Manufacturing method of semiconductor device

ABSTRACT

A method for manufacturing a semiconductor device includes the steps of forming a first insulating film over a first gate electrode over a substrate while heated at a temperature higher than or equal to 450° C. and lower than the strain point of the substrate, forming a first oxide semiconductor film over the first insulating film, adding oxygen to the first oxide semiconductor film and then forming a second oxide semiconductor film over the first oxide semiconductor film, and performing heat treatment so that part of oxygen contained in the first oxide semiconductor film is transferred to the second oxide semiconductor film.

TECHNICAL FIELD

The present invention relates to an object, a process (including amethod and a manufacturing method), a machine, a manufacture, or acomposition of matter. In particular, one embodiment of the presentinvention relates to a semiconductor device, a display device, alight-emitting device, a driving method thereof, a manufacturing methodthereof, or the like. In particular, one embodiment of the presentinvention relates to a semiconductor device, a display device, a memorydevice, a light-emitting device, or the like including an oxidesemiconductor.

In this specification, the category of semiconductor devices includesall devices that can function by utilizing electronic characteristics ofa semiconductor; for example, a semiconductor circuit is included in thecategory of semiconductor devices. Moreover, an electro-optic device, adisplay device, an electric appliance, or the like includes asemiconductor device in some cases.

BACKGROUND ART

Transistors used for most flat panel displays typified by a liquidcrystal display device and a light-emitting display device are formedusing silicon semiconductors such as amorphous silicon, single crystalsilicon, and polycrystalline silicon provided over glass substrates. Thetransistors formed using such silicon semiconductors are also used inintegrated circuits (ICs) and the like.

In recent years, attention has been drawn to a technique in which,instead of a silicon semiconductor, a metal oxide exhibitingsemiconductor characteristics is used for transistors. Note that in thisspecification, a metal oxide exhibiting semiconductor characteristics isreferred to as an oxide semiconductor.

For example, disclosed is a technique in which a transistor ismanufactured using zinc oxide or an In—Ga—Zn oxide as an oxidesemiconductor and the transistor is used as a switching element or thelike of a pixel of a display device (see Patent Documents 1 and 2).

REFERENCES Patent Documents

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

DISCLOSURE OF INVENTION

In a transistor using an oxide semiconductor, oxygen vacancies, whichmay cause localized states in an oxide semiconductor film, lead to poorelectrical characteristics of the transistor.

In view of this, an object of one embodiment of the present invention isto improve the electrical characteristics of a semiconductor deviceusing an oxide semiconductor. Another object of one embodiment of thepresent invention is to improve the reliability of a semiconductordevice using an oxide semiconductor. Another object of one embodiment ofthe present invention is to reduce oxygen vacancies in an oxidesemiconductor. Another object of one embodiment of the present inventionis to prevent a transistor from becoming normally-on. Another object ofone embodiment of the present invention is to control change, variation,or decrease in the threshold voltage of a transistor. Another object ofone embodiment of the present invention is to provide a transistorhaving a low off-state current. Another object of one embodiment of thepresent invention is to provide a novel semiconductor device or thelike.

Note that the description of these objects does not disturb theexistence of other objects. Note that there is no need to achieve all ofthese objects in one embodiment of the present invention. Other objectswill be apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention is a transistor including afirst oxide semiconductor film, a second oxide semiconductor film, apair of electrodes in contact with the second oxide semiconductor film,and a third oxide semiconductor film in contact with the second oxidesemiconductor film and the pair of electrodes, in which oxygen is addedto the first oxide semiconductor film or the third oxide semiconductorfilm to reduce oxygen vacancies. Further, the oxygen is diffused intothe second oxide semiconductor film by heat treatment or the like, sothat oxygen vacancies in the second oxide semiconductor film arereduced.

One embodiment of the present invention is a method for manufacturing asemiconductor device including the following steps. A first insulatingfilm is formed over a first gate electrode over a substrate while heatedat a temperature higher than or equal to 450° C. and lower than thestrain point of the substrate. A first oxide semiconductor film isformed over the first insulating film. After oxygen is added to thefirst oxide semiconductor film, a second oxide semiconductor film isformed over the first oxide semiconductor film. First heat treatment isperformed so that part of oxygen contained in the first oxidesemiconductor film is transferred to the second oxide semiconductorfilm. Next, part of the first insulating film, part of the first oxidesemiconductor film to which oxygen has been added, and part of thesecond oxide semiconductor film are etched to form a first gateinsulating film having a projection, the etched first oxidesemiconductor film, and the etched second oxide semiconductor film.Then, a pair of electrodes is formed over the etched second oxidesemiconductor film, and a third oxide semiconductor film is formed overthe etched second oxide semiconductor film and the pair of electrodes.Next, a second gate insulating film is formed over the third oxidesemiconductor film, and a second gate electrode is formed over thesecond gate insulating film.

One embodiment of the present invention is a method for manufacturing asemiconductor device including the following steps. A first insulatingfilm is formed over a first gate electrode over a substrate while heatedat a temperature higher than or equal to 450° C. and lower than thestrain point of the substrate. A first oxide semiconductor film isformed over the first insulating film, and a second oxide semiconductorfilm is formed over the first oxide semiconductor film. Then, part ofthe first insulating film, part of the first oxide semiconductor film,and part of the second oxide semiconductor film are etched to form afirst gate insulating film having a projection, the etched first oxidesemiconductor film, and the etched second oxide semiconductor film.Next, a pair of electrodes is formed over the etched second oxidesemiconductor film, and a third oxide semiconductor film is formed overthe etched second oxide semiconductor film and the pair of electrodes.After oxygen is added to the third oxide semiconductor film, first heattreatment is performed so that part of oxygen contained in the thirdoxide semiconductor film is transferred to the etched second oxidesemiconductor film. Then, a second gate insulating film is formed overthe third oxide semiconductor film to which oxygen has been added, and asecond gate electrode is formed over the second gate insulating film.

The temperature of the first heat treatment is preferably lower than atemperature at which the first insulating film is formed.

Note that by adding oxygen to the first oxide semiconductor film and/orthe third oxide semiconductor film and then performing heat treatment,oxygen vacancies in the first oxide semiconductor film and/or the thirdoxide semiconductor film can be reduced.

Note that the second oxide semiconductor film is an oxide semiconductorfilm containing In or Ga and is typically an In—Ga oxide film, an In—Znoxide film, an In—Mg oxide film, a Zn—Mg oxide film, or an In-M-Zn oxidefilm (M is Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd). Note that theelement M is a metal element whose strength of bonding with oxygen ishigher than that of In.

The first oxide semiconductor film and the third oxide semiconductorfilm are typically each an In—Ga oxide film, an In—Zn oxide film, anIn—Mg oxide film, a Zn—Mg oxide film, or an In-M-Zn oxide film (M is Al,Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd), and has the energy level at theconduction band minimum that is closer to a vacuum level than that ofthe second oxide semiconductor film is. Typically, a difference betweenthe energy level at the conduction band minimum of the second oxidesemiconductor film and the energy level at the conduction band minimumof each of the first and third oxide semiconductor films is greater thanor equal to 0.05 eV, greater than or equal to 0.07 eV, greater than orequal to 0.1 eV, or greater than or equal to 0.2 eV and also less thanor equal to 2 eV, less than or equal to 1 eV, less than or equal to 0.5eV, or less than or equal to 0.4 eV. Note that the difference betweenthe vacuum level and the energy at the conduction band minimum isreferred to as electron affinity.

In the case where the first and third oxide semiconductor films and thesecond oxide semiconductor film are each an In-M-Zn oxide film (M is Al,Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd), the proportion of M atoms (M isAl, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd) in each of the first and thirdoxide semiconductor films is higher than that in the second oxidesemiconductor film. Typically, the proportion of M in each of the firstand third oxide semiconductor films is higher than or equal to 1.5times, preferably higher than or equal to twice, further preferablyhigher than or equal to three times as high as that in the second oxidesemiconductor film.

As a method for adding oxygen to the first oxide semiconductor film orthe third oxide semiconductor film, an ion implantation method, an iondoping method, plasma treatment, or the like can be used. The oxygenadded to the first oxide semiconductor film or the third oxidesemiconductor film is at least one kind selected from oxygen radicals,oxygen atoms, oxygen atomic ions, oxygen molecular ions, and the like.

According to one embodiment of the present invention, the electricalcharacteristics of a semiconductor device using an oxide semiconductorcan be improved. According to one embodiment of the present invention,the reliability of a semiconductor device using an oxide semiconductorcan be improved. Furthermore, according to one embodiment of the presentinvention, a novel semiconductor device or the like can be provided.Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily have all the effects listed above. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device;

FIG. 2 shows the electrical characteristics of the semiconductor device;

FIGS. 3A to 3D are cross-sectional views illustrating one embodiment ofa method for manufacturing the semiconductor device;

FIGS. 4A to 4C are cross-sectional views illustrating one embodiment ofa method for manufacturing the semiconductor device;

FIGS. 5A and 5B each show a band structure of a transistor;

FIGS. 6A to 6C are cross-sectional views illustrating one embodiment ofa semiconductor device;

FIGS. 7A to 7D are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device;

FIGS. 8A to 8C are cross-sectional views illustrating one embodiment ofa method for manufacturing the semiconductor device;

FIG. 9 is a cross-sectional view illustrating one embodiment of asemiconductor device;

FIGS. 10A to 10C are a top view and cross-sectional views illustratingone embodiment of a semiconductor device;

FIGS. 11A to 11C are a top view and cross-sectional views illustratingone embodiment of a semiconductor device;

FIGS. 12A to 12D are cross-sectional views illustrating one embodimentof a method for manufacturing a semiconductor device;

FIGS. 13A to 13C are cross-sectional views illustrating one embodimentof the method for manufacturing the semiconductor device;

FIGS. 14A to 14C are cross-sectional views illustrating one embodimentof a method for manufacturing a semiconductor device;

FIGS. 15A to 15C are cross-sectional views each illustrating oneembodiment of a semiconductor device;

FIGS. 16A and 16B illustrate the relationship between formation energyand transition levels, and the electron configurations of defects;

FIG. 17 illustrates a change in the Fermi level and a change in thecharge states of defects;

FIG. 18 shows a crystalline model of c-SiO₂;

FIG. 19 shows a model in which NO₂ is introduced into an interstitialsite of a c-SiO₂ model;

FIG. 20 shows a model in which N₂O is introduced into an interstitialsite of a c-SiO₂ model;

FIG. 21 shows a model in which NO is introduced into an interstitialsite of a c-SiO₂ model;

FIG. 22 shows a model in which an N atom is introduced into aninterstitial site of a c-SiO₂ model;

FIG. 23 is a band diagram;

FIGS. 24A and 24B each show a model of a cluster structure;

FIG. 25 shows a mechanism of a phenomenon in which the threshold voltageof a transistor is shifted in the positive direction;

FIGS. 26A and 26B are a cross-sectional view and a circuit diagramillustrating one embodiment of a semiconductor device;

FIG. 27 illustrates a data writing operation and a data readingoperation of a memory cell;

FIGS. 28A to 28C are a block diagram and circuit diagrams illustrating adisplay device;

FIG. 29 illustrates a display module;

FIGS. 30A to 30F illustrate electronic devices of one embodiment of thepresent invention;

FIG. 31 shows the calculation results of oxygen concentration;

FIG. 32 shows the calculation results of oxygen concentration;

FIGS. 33A to 33I illustrate fabrication methods of samples;

FIGS. 34A and 34B show the results of ESR measurement;

FIGS. 35A to 35F illustrate fabrication methods of samples;

FIGS. 36A and 36B show the results of ESR measurement;

FIG. 37 shows the results of ESR measurement;

FIG. 38 shows the results of TDS analysis;

FIG. 39 shows the results of SIMS analysis;

FIG. 40 shows the measurement results of the V_(g)−Id characteristics ofa transistor;

FIG. 41 shows the measurement results of the V_(g)−Id characteristics ofa transistor;

FIGS. 42A and 42B show the relationship between stress time and each ofthe amount of change in threshold voltage and the amount of change inshift value;

FIG. 43 is a cross-sectional view illustrating one embodiment of asemiconductor device;

FIGS. 44A and 44B are cross-sectional views each illustrating oneembodiment of a semiconductor device;

FIGS. 45A and 45B show ESR spectra of NO₂ and Si—N—Si;

FIG. 46 shows the results of SIMS analysis;

FIGS. 47A to 47D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of theCAAC-OS;

FIGS. 48A to 48D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS;

FIGS. 49A to 49C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD;

FIGS. 50A and 50B show electron diffraction patterns of a CAAC-OS; and

FIG. 51 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. Note that the present invention is notlimited to the following description. It will be readily appreciated bythose skilled in the art that modes and details of the present inventioncan be changed in various ways without departing from the spirit andscope of the present invention. Therefore, the present invention shouldnot be construed as being limited to the following description of theembodiments. In addition, in the following embodiments and examples, thesame portions or portions having similar functions are denoted by thesame reference numerals or the same hatching patterns in differentdrawings, and description thereof will not be repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such a scale.

In addition, terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

The functions of “source” and “drain” may be switched in the case where,for example, the direction of a current flow is changed during circuitoperation. Therefore, the terms “source” and “drain” can be used todenote the drain and the source, respectively, in this specification.

The term “parallel” indicates that the angle formed between two straightlines is greater than or equal to −10° and less than or equal to 10°,and accordingly also includes the case where the angle is greater thanor equal to −5° and less than or equal to 5°. The term “substantiallyparallel” indicates that the angle formed between two straight lines isgreater than or equal to −30° and less than or equal to 30°. The term“perpendicular” indicates that the angle formed between two straightlines is greater than or equal to 80° and less than or equal to 100°,and accordingly also includes the case where the angle is greater thanor equal to 85° and less than or equal to 95°. The term “substantiallyperpendicular” indicates that the angle formed between two straightlines is greater than or equal to 60° and less than or equal to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

A voltage refers to a difference between potentials of two points, and apotential refers to electrostatic energy (electric potential energy) ofa unit charge at a given point in an electrostatic field. Note that ingeneral, a difference between a potential of one point and a referencepotential (e.g., a ground potential) is merely called a potential or avoltage, and a potential and a voltage are used as synonymous words inmany cases. Thus, in this specification, a potential may be rephrased asa voltage and a voltage may be rephrased as a potential unless otherwisespecified.

A transistor including an oxide semiconductor film is an n-channeltransistor; therefore, in this specification, a transistor that can beregarded as having no drain current flowing therein when a gate voltageis 0 V is defined as a transistor having normally-off characteristics.In contrast, a transistor that can be regarded as having a drain currentflowing therein when the gate voltage is 0 V is defined as a transistorhaving normally-on characteristics.

Note that the channel length refers to, for example, the distancebetween a source (a source region or a source electrode) and a drain (adrain region or a drain electrode) in a region where an oxidesemiconductor film (or a portion where a current flows in an oxidesemiconductor film when a transistor is on) and a gate electrode overlapwith each other or a region where a channel is formed in a top view ofthe transistor. In one transistor, channel lengths in all regions arenot necessarily the same. In other words, the channel length of onetransistor is not limited to one value in some cases. Therefore, in thisspecification, the channel length is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

The channel width refers to, for example, the length of a portion wherea source and a drain face each other in a region where an oxidesemiconductor film (or a portion where a current flows in an oxidesemiconductor film when a transistor is on) and a gate electrode overlapwith each other, or a region where a channel is formed. In onetransistor, channel widths in all regions are not necessarily the same.In other words, the channel width of one transistor is not limited toone value in some cases. Therefore, in this specification, the channelwidth is any one of values, the maximum value, the minimum value, or theaverage value in a region where a channel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of an oxidesemiconductor film is higher than the proportion of a channel regionformed in a top surface of the oxide semiconductor film in some cases.In that case, an effective channel width obtained when a channel isactually formed is greater than an apparent channel width shown in thetop view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example, toestimate an effective channel width from a design value, it is necessaryto assume that the shape of an oxide semiconductor film is known.Therefore, in the case where the shape of an oxide semiconductor film isnot known accurately, it is difficult to measure an effective channelwidth accurately.

Thus, in this specification, an apparent channel width, which is thelength of a portion where a source and a drain face each other in aregion where an oxide semiconductor film and a gate electrode overlapwith each other in a top view of a transistor, is referred to as a“surrounded channel width (SCW)” in some cases. Further, in thisspecification, in the case where the term “channel width” is simplyused, it may denote a surrounded channel width or an apparent channelwidth. Alternatively, in this specification, in the case where the term“channel width” is simply used, it denotes an effective channel width insome cases. Note that the values of a channel length, a channel width,an effective channel width, an apparent channel width, a surroundedchannel width, and the like can be determined by obtaining and analyzinga cross-sectional TEM image and the like.

Note that in the case where the electric field mobility, the currentvalue per channel width, and the like of a transistor are obtained bycalculation, a surrounded channel width may be used for the calculation.The value obtained in that case is sometimes different from the oneobtained from the calculation using an effective channel width.

Embodiment 1

The threshold voltage of a transistor using an oxide semiconductor filmincluding oxygen vacancies easily shifts in the negative direction, andsuch a transistor tends to be normally-on. This is because electriccharges are generated owing to oxygen vacancies in the oxidesemiconductor film, and the resistance is reduced. In addition, atransistor using an oxide semiconductor film including oxygen vacancieshas such a problem that the electrical characteristics, typically, thethreshold voltage, are changed with time or changed by a stress test(typically, a gate bias-temperature (BT) stress test under lightirradiation). In this embodiment, a highly reliable semiconductor devicewith a small change in threshold voltage and a manufacturing methodthereof will be described. Further, a semiconductor device withexcellent electrical characteristics and a manufacturing method thereofwill be described.

<Structure Example of Semiconductor Device>

In this embodiment, a method for manufacturing a top-gate transistor isdescribed.

FIGS. 1A to 1C are a top view and cross-sectional views of a transistor100 of a semiconductor device. FIG. 1A is a top view of the transistor100, FIG. 1B is a cross-sectional view taken along dashed-dotted lineA-B in FIG. 1A, and FIG. 1C is a cross-sectional view taken alongdashed-dotted line C-D in FIG. 1A. Note that in FIG. 1A, a substrate101, a gate insulating film 105, an oxide semiconductor film 107, anoxide semiconductor film 115, a gate insulating film 117, an insulatingfilm 121, an insulating film 123, and the like are not illustrated forsimplicity.

FIG. 1B is a cross-sectional view in the channel length direction of thetransistor 100 and FIG. 1C is a cross-sectional view in the channelwidth direction of the transistor 100.

The transistor 100 illustrated in FIGS. 1A to 1C is provided over thesubstrate 101. The transistor 100 includes a gate electrode 103 over thesubstrate 101, the gate insulating film 105 over the substrate 101 andthe gate electrode 103, the oxide semiconductor film 107 in contact withthe gate insulating film 105, an oxide semiconductor film 111 in contactwith the oxide semiconductor film 107, a pair of electrodes 113 a and113 b in contact with at least a top surface and a side surface of theoxide semiconductor film 111 and a side surface of the oxidesemiconductor film 107, the oxide semiconductor film 115 in contact withthe oxide semiconductor film 111 and the pair of electrodes 113 a and113 b, the gate insulating film 117 overlapping with the oxidesemiconductor film 111 with the oxide semiconductor film 115 positionedtherebetween, and a gate electrode 119 being in contact with the gateinsulating film 117 and overlapping with the oxide semiconductor film111 with the oxide semiconductor film 115 and the gate insulating film117 positioned therebetween. Furthermore, the transistor 100 may includethe insulating film 121 that covers the pair of electrodes 113 a and 113b, the oxide semiconductor film 115, the gate insulating film 117, andthe gate electrode 119 and the insulating film 123 that covers theinsulating film 121.

Note that at least part (or the whole) of the electrode 113 a (and/orthe electrode 113 b) is provided on at least part (or the whole) of asurface, a side surface, a top surface, and/or a bottom surface of asemiconductor film such as the oxide semiconductor film 107 (and/or theoxide semiconductor film 111).

Alternatively, at least part (or the whole) of the electrode 113 a(and/or the electrode 113 b) is in contact with at least part (or thewhole) of a surface, a side surface, a top surface, and/or a bottomsurface of a semiconductor film such as the oxide semiconductor film 107(and/or the oxide semiconductor film 111). Alternatively, at least part(or the whole) of the electrode 113 a (and/or the electrode 113 b) is incontact with at least part (or the whole) of a semiconductor film suchas the oxide semiconductor film 107 (and/or the oxide semiconductor film111).

Alternatively, at least part (or the whole) of the electrode 113 a(and/or the electrode 113 b) is electrically connected to at least part(or the whole) of a surface, a side surface, a top surface, and/or abottom surface of a semiconductor film such as the oxide semiconductorfilm 107 (and/or the oxide semiconductor film 111). Alternatively, atleast part (or the whole) of the electrode 113 a (and/or the electrode113 b) is electrically connected to part (or the whole) of asemiconductor film such as the oxide semiconductor film 107 (and/or theoxide semiconductor film 111).

Alternatively, at least part (or the whole) of the electrode 113 a(and/or the electrode 113 b) is positioned close to at least part (orthe whole) of a surface, a side surface, a top surface, and/or a bottomsurface of a semiconductor film such as the oxide semiconductor film 107(and/or the oxide semiconductor film 111). Alternatively, at least part(or the whole) of the electrode 113 a (and/or the electrode 113 b) ispositioned close to part (or the whole) of a semiconductor film such asthe oxide semiconductor film 107 (and/or the oxide semiconductor film111).

Alternatively, at least part (or the whole) of the electrode 113 a(and/or the electrode 113 b) is positioned beside at least part (or thewhole) of a surface, a side surface, a top surface, and/or a bottomsurface of a semiconductor film such as the oxide semiconductor film 107(and/or the oxide semiconductor film 111). Alternatively, at least part(or the whole) of the electrode 113 a (and/or the electrode 113 b) ispositioned beside part (or the whole) of a semiconductor film such asthe oxide semiconductor film 107 (and/or the oxide semiconductor film111).

Alternatively, at least part (or the whole) of the electrode 113 a(and/or the electrode 113 b) is positioned diagonally above at leastpart (or the whole) of a surface, a side surface, a top surface, and/ora bottom surface of a semiconductor film such as the oxide semiconductorfilm 107 (and/or the oxide semiconductor film 111). Alternatively, atleast part (or the whole) of the electrode 113 a (and/or the electrode113 b) is positioned diagonally above part (or the whole) of asemiconductor film such as the oxide semiconductor film 107 (and/or theoxide semiconductor film 111).

Alternatively, at least part (or the whole) of the electrode 113 a(and/or the electrode 113 b) is positioned above at least part (or thewhole) of a surface, a side surface, a top surface, and/or a bottomsurface of a semiconductor film such as the oxide semiconductor film 107(and/or the oxide semiconductor film 111). Alternatively, at least part(or the whole) of the electrode 113 a (and/or the electrode 113 b) ispositioned above part (or the whole) of a semiconductor film such as theoxide semiconductor film 107 (and/or the oxide semiconductor film 111).

The transistor 100 includes two gate electrodes. One of them has afunction of controlling the on/off state of the transistor 100. Theother has a function of controlling the threshold voltage of thetransistor 100. When a voltage that makes the threshold voltage of thetransistor 100 positive is applied to one of the gate electrodes, thetransistor 100 can have normally-off characteristics.

The gate insulating film 105 included in the transistor 100 has aprojection. The oxide semiconductor films 107 and 111 are formed in thisorder over the projection. Hence, in the channel width direction, thegate electrode 119 faces side surfaces of the oxide semiconductor films107 and 111 with the gate insulating film 117 positioned therebetween asillustrated in FIG. 1C. In other words, when a voltage is applied to thegate electrode 119, the oxide semiconductor films 107 and 111 aresurrounded by the electric field of the gate electrode 119 in thechannel width direction. The transistor structure in which an oxidesemiconductor film is surrounded by the electric field of the gateelectrode 119 is referred to as a surrounded channel (s-channel)structure. In the transistor with the s-channel structure, a channel isformed in the whole oxide semiconductor film 111 (bulk) in the on state,increasing the on-state current. In the off state, the entire region ofthe channel region formed in the oxide semiconductor film 111 isdepleted, which allows the off-state current to be further reduced.

Components of the transistor 100 are described below.

Although there is no particular limitation on a material and the like ofthe substrate 101, it is necessary that the substrate have heatresistance high enough to withstand at least heat treatment performedlater. For example, a glass substrate, a ceramic substrate, a quartzsubstrate, a sapphire substrate, or the like may be used as thesubstrate 101. Alternatively, a single crystal semiconductor substrateor a polycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like, a compound semiconductor substrate made of silicongermanium or the like, a silicon on insulator (SOI) substrate, or thelike may be used as the substrate 101. Furthermore, any of thesesubstrates provided with a semiconductor element may be used as thesubstrate 101. Alternatively, gallium arsenide, aluminum galliumarsenide, indium gallium arsenide, gallium nitride, indium phosphide,silicon germanium, or the like which can be employed for ahigh-electron-mobility transistor (HEMT) may be used as the material ofthe substrate 101. By using any of these semiconductors for thesubstrate 101, the transistor 100 suited to high speed operation can befabricated. The substrate 101 is not limited to a simple supportingsubstrate, and may be a substrate where a device such as a transistor isformed. In this case, at least one of the gate electrode, the sourceelectrode, and the drain electrode of the transistor 100 may beelectrically connected to the device.

Note that a flexible substrate may also be used as the substrate 101. Inthe case where a flexible substrate is used, a semiconductor elementsuch as a transistor or a capacitor may be directly formed over theflexible substrate, or the semiconductor element may be formed over amanufacturing substrate and then separated and transferred onto theflexible substrate. To perform separation and transfer of thesemiconductor element from the manufacturing substrate to the flexiblesubstrate, a separation layer may be provided between the manufacturingsubstrate and the semiconductor element.

The gate electrode 103 has a function of controlling the thresholdvoltage of the transistor 100. The gate electrode 103 can be formedusing a metal element selected from aluminum, chromium, copper,tantalum, titanium, molybdenum, manganese, and tungsten; an alloycontaining any of these metal elements as a component; an alloycontaining any of these metal elements in combination; or the like.Furthermore, one or more metal elements selected from manganese andzirconium may be used. Moreover, the gate electrode 103 may have asingle-layer structure or a stacked structure of two or more layers. Forexample, the gate electrode 103 can have a single-layer structure of analuminum film containing silicon, a single-layer structure of a copperfilm containing manganese, a two-layer structure in which a titaniumfilm is stacked over an aluminum film, a two-layer structure in which acopper film is stacked over a copper-magnesium alloy film, a two-layerstructure in which a titanium film is stacked over a titanium nitridefilm, a two-layer structure in which a tungsten film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a tantalum nitride film or a tungsten nitride film, athree-layer structure in which a titanium film, an aluminum film, and atitanium film are stacked in this order, or a three-layer structure inwhich a copper-magnesium alloy film, a copper film, and acopper-magnesium alloy film are stacked in this order. Alternatively, analloy film or a nitride film which contains aluminum and one or moreelements selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium may be used.

The gate electrode 103 can also be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. The gate electrode 103 can have a stacked structure formed usingthe above-described light-transmitting conductive material and theabove-described metal element.

Note that it is preferable that element(s) included in the gateelectrode 103 be not diffused into the gate insulating film 105. Forexample, when an alloy film that contains one or more elements selectedfrom tungsten, tantalum, molybdenum, copper, titanium, and aluminum, analloy film to which a small amount of element is added, or the like isused as the gate electrode 103, it is hard for the element(s) includedin the gate electrode 103 to be diffused into the gate insulating film105.

As the gate electrode 103, an In—Ga—Zn oxynitride film, an In—Snoxynitride film, an In—Ga oxynitride film, an In—Zn oxynitride film, ametal oxynitride film (e.g., SnON, InON), a metal nitride film (e.g.,InN, ZnN), or the like may be provided. For example, in the case ofusing an In—Ga—Zn oxynitride film, an In—Ga—Zn oxynitride film whosenitrogen concentration is higher than at least the nitrogenconcentration of the oxide semiconductor film 111, specifically, anIn—Ga—Zn oxynitride film whose nitrogen concentration is higher than orequal to 7 at. % is used. In the case where the gate electrode 103 isformed using any of the above-described metal oxynitride films or themetal nitride films, it is preferable to provide a protective film thatprevents diffusion of metal, oxygen, or nitrogen over the gate electrode103. Examples of the protective film are an alloy film containing one ormore elements selected from tungsten, tantalum, molybdenum, copper,titanium, and aluminum, and an alloy film to which a small amount ofelement is added.

The gate insulating film 105 has a function of a base film of the oxidesemiconductor film 107. As the gate insulating film 105, a single layeror a stacked layer of, for example, a silicon oxide film, a siliconoxynitride film, a silicon nitride oxide film, a silicon nitride film,an aluminum oxide film, a hafnium oxide film, a gallium oxide film, or aGa—Zn metal oxide film can be provided. The gate insulating film 105preferably includes a small number of defects or impurities in order toreduce a shift in the electrical characteristics of the transistor suchas the threshold voltage.

It is possible to prevent outward diffusion of oxygen from the oxidesemiconductor film 107 and entry of hydrogen, water, or the like intothe oxide semiconductor film 107 from the outside by providing aninsulating film having a blocking effect against oxygen, hydrogen,water, and the like as the gate insulating film 105. Examples of theinsulating film having a blocking effect against oxygen, hydrogen,water, and the like include an aluminum oxide film, an aluminumoxynitride film, a gallium oxide film, a gallium oxynitride film, anyttrium oxide film, an yttrium oxynitride film, a hafnium oxide film,and a hafnium oxynitride film.

The gate insulating film 105 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, yttrium oxide, or aluminum oxide,so that the gate leakage current of the transistor can be reduced.

The thickness of the gate insulating film 105 is greater than or equalto 5 nm and less than or equal to 400 nm, greater than or equal to 5 nmand less than or equal to 300 nm, or greater than or equal to 10 nm andless than or equal to 50 nm. By reducing the thickness of the gateinsulating film 105, the voltage applied to the gate electrode 103 canbe lowered, reducing the power consumption of the semiconductor device.

The gate insulating film 105 can be formed using an oxide insulatingfilm with a low content of nitrogen oxide and a low density of defectstates. The oxide insulating film with a low content of nitrogen oxideand a low density of defect states is, specifically, an oxide insulatingfilm having a low density of defect states located 4.6 eV or more and 8eV or less lower than a vacuum level, that is, an oxide film having alow density of defect states attributed to nitrogen oxide. As the oxideinsulating film with a low content of nitrogen oxide and a low densityof defect states, a silicon oxynitride film that releases littlenitrogen oxide, a silicon oxide film that releases little nitrogenoxide, an aluminum oxynitride film that releases little nitrogen oxide,an aluminum oxide film that releases little nitrogen oxide, or the likecan be used.

Nitrogen oxide (NO_(x); x is greater than or equal to 0 and less than orequal to 2, preferably greater than or equal to 1 and less than or equalto 2), typically NO₂ or NO, forms levels in, for example, the gateinsulating film 105. The level is positioned in the energy gap of theoxide semiconductor film 107. Therefore, when nitrogen oxide is diffusedinto the interface between the gate insulating film 105 and the oxidesemiconductor film 107, an electron is trapped by the level on the gateinsulating film 105 side of the oxide semiconductor film 107. As aresult, the trapped electron remains in the vicinity of the interfacebetween the gate insulating film 105 and the oxide semiconductor film107; thus, the threshold voltage of the transistor is shifted in thepositive direction.

By using an oxide insulating film with a low content of nitrogen oxideand a low density of defect states for the gate insulating film 105, ashift in the threshold voltage of the transistor can be reduced, whichleads to a smaller change in the electrical characteristics of thetransistor.

Note that in an electron spin resonance (ESR) spectrum at 100 K or lowerof the gate insulating film 105, by heat treatment in a manufacturingprocess of the transistor, typically heat treatment at a temperaturehigher than or equal to 300° C. and lower than the strain point of thesubstrate, a first signal that appears at a g-factor of greater than orequal to 2.037 and less than or equal to 2.039, a second signal thatappears at a g-factor of greater than or equal to 2.001 and less than orequal to 2.003, and a third signal that appears at a g-factor of greaterthan or equal to 1.964 and less than or equal to 1.966 are not observed.The split width of the first and second signals and the split width ofthe second and third signals that are obtained by ESR measurement usingan X-band are each approximately 5 mT. The sum of the spin densities ofthe first signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is lower than thedetection limit, typically lower than or equal to 1×10¹⁷ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 correspond to signals attributed to nitrogendioxide. Accordingly, the lower the sum of the spin densities of thefirst signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is, the lower thecontent of nitrogen oxide in the oxide insulating film is.

After heat treatment in the manufacturing process of the transistor,typically heat treatment at a temperature higher than or equal to 300°C. and lower than the strain point of the substrate, the oxideinsulating film with a low content of nitrogen oxide and a low densityof defect states has a nitrogen concentration measured by secondary ionmass spectrometry (SIMS) of less than 2×10²⁰ atoms/cm³, less than 7×10¹⁹atoms/cm³, or less than 2×10¹⁹ atoms/cm³. The content of nitrogen oxidein the gate insulating film 105 can be reduced as the formationtemperature of the gate insulating film 105 rises. The gate insulatingfilm 105 is preferably formed at a temperature higher than or equal to450° C. and lower than the strain point of the substrate, higher than orequal to 500° C. and lower than strain point of the substrate, or higherthan or equal to 500° C. and lower than or equal to 550° C.

The oxide semiconductor film 111 is an oxide semiconductor filmcontaining In or Ga and typically an In—Ga oxide film, an In—Zn oxidefilm, an In—Mg oxide film, a Zn—Mg oxide film, or an In-M-Zn oxide film(M is Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd).

Note that in the case where the oxide semiconductor film 111 is anIn-M-Zn oxide film, the proportions of In and M when the summation of Inand M is assumed to be 100 atomic % are preferably as follows: theatomic percentage of In is greater than or equal to 25 atomic % and theatomic percentage of M is less than 75 atomic %, or further preferably,the atomic percentage of In is greater than or equal to 34 atomic % andthe atomic percentage of M is less than 66 atomic %.

The indium and gallium contents in the oxide semiconductor film 111 canbe compared with each other by time-of-flight secondary ion massspectrometry (TOF-SIMS), X-ray photoelectron spectrometry (XPS), orinductively coupled plasma mass spectrometry (ICP-MS).

Since the oxide semiconductor film 111 has an energy gap of 2 eV ormore, preferably 2.5 eV or more, further preferably 3 eV or more, theoff-state current of the transistor 100 can be low.

The thickness of the oxide semiconductor film 111 is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm, further preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

The oxide semiconductor film 107 and the oxide semiconductor film 115are oxide semiconductor films containing one or more elements that areincluded in the oxide semiconductor film 111. Therefore, interfacescattering is unlikely to occur at the interface between the oxidesemiconductor film 111 and each of the oxide semiconductor films 107 and115. Thus, the transistor 100 can have a high field-effect mobilitybecause the movement of carriers is not hindered at the interfaces.

The oxide semiconductor film 107 and the oxide semiconductor film 115are typically each a Ga oxide film, an In—Ga oxide film, an In—Zn oxidefilm, an In—Mg oxide film, a Zn—Mg oxide film, or an In-M-Zn oxide film(M is Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd), and has the energylevel at the conduction band minimum that is closer to a vacuum levelthan that of the oxide semiconductor film 111 is. Typically, adifference between the energy level at the conduction band minimum ofthe oxide semiconductor film 111 and the energy level at the conductionband minimum of each of the oxide semiconductor films 107 and 115 isgreater than or equal to 0.05 eV, greater than or equal to 0.07 eV,greater than or equal to 0.1 eV, or greater than or equal to 0.2 eV andalso less than or equal to 2 eV, less than or equal to 1 eV, less thanor equal to 0.5 eV, or less than or equal to 0.4 eV. That is, thedifference between the electron affinity of the oxide semiconductor film111 and the electron affinity of each of the oxide semiconductor films107 and 115 is greater than or equal to 0.05 eV, greater than or equalto 0.07 eV, greater than or equal to 0.1 eV, or greater than or equal to0.2 eV and also less than or equal to 2 eV, less than or equal to 1 eV,less than or equal to 0.5 eV, or less than or equal to 0.4 eV. Note thatthe electron affinity refers to the energy gap between the vacuum leveland the conduction band minimum.

When the oxide semiconductor films 107 and 115 contain a larger amountof Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd in an atomic ratio than theamount of In in an atomic ratio, any of the following effects may beobtained.

(1) The energy gap of each of the oxide semiconductor films 107 and 115is widened.

(2) The electron affinity of each of the oxide semiconductor films 107and 115 is reduced.

(3) Impurities from the outside are blocked.

(4) An insulating property of each of the oxide semiconductor films 107and 115 is higher than that of the oxide semiconductor film 111.

(5) Oxygen vacancies are less likely to be generated in the oxidesemiconductor films 107 and 115 because Al, Ti, Ga, Y, Zr, Sn, La, Ce,Mg, and Nd are metal elements that can be strongly bonded to oxygen.

When an In-M-Zn oxide is used for the oxide semiconductor films 107 and115, the proportions of In and M, not taking Zn and O intoconsideration, are preferably as follows: the atomic percentage of In isless than 50 atomic % and the atomic percentage of M is greater than orequal to 50 atomic %; further preferably, the atomic percentage of In isless than 25 atomic % and the atomic percentage of M is greater than orequal to 75 atomic %.

Further, in the case where the oxide semiconductor films 107, 111, and115 are each an In-M-Zn oxide film (M is Al, Ti, Ga, Y, Zr, Sn, La, Ce,Mg, or Nd), the proportion of M atoms (M is Al, Ti, Ga, Y, Zr, Sn, La,Ce, Mg, or Nd) in each of the oxide semiconductor films 107 and 115 ishigher than that in the oxide semiconductor film 111. Typically, theproportion of M in each of the oxide semiconductor films 107 and 115 ishigher than or equal to 1.5 times, preferably higher than or equal totwice, further preferably higher than or equal to three times as high asthat in the oxide semiconductor film 111. Any of the above-describedelements represented by M is more strongly bonded to oxygen than indiumis, and thus has a function of suppressing generation of oxygenvacancies in the oxide semiconductor films 107 and 115. That is, oxygenvacancies are less likely to be generated in the oxide semiconductorfilms 107 and 115 than in the oxide semiconductor film 111.

In the case where the oxide semiconductor film 111 is an In-M-Zn oxidefilm (M is Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd) and a target havingthe atomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used forforming the oxide semiconductor film 111, x₁/y₁ is preferably greaterthan or equal to ⅓ and less than or equal to 6, further preferablygreater than or equal to 1 and less than or equal to 6, and z₁/y₁ ispreferably greater than or equal to ⅓ and less than or equal to 6,further preferably greater than or equal to 1 and less than or equal to6. Note that when z₁/y₁ is greater than or equal to 1 and less than orequal to 6, a c-axis aligned crystalline oxide semiconductor (CAAC-OS)film as the oxide semiconductor film 111 can be easily formed. Typicalexamples of the atomic ratio of In to M and Zn of the target are 1:1:1,1:1:1.2, 2:1:1.5, 2:1:2.3, 2:1:3, 3:1:2, and 4:2:4.1.

In the case where the oxide semiconductor film 107 and the oxidesemiconductor film 115 are each an In-M-Zn oxide film (M is Al, Ti, Ga,Y, Zr, Sn, La, Ce, Mg, or Nd) and a target having the atomic ratio ofmetal elements of In:M:Zn=x₂:y₂:z₂ is used for forming the oxidesemiconductor films 107 and 115, x₂/y₂ is preferably less than x₁/y₁,and z₂/y₂ is preferably greater than or equal to ⅓ and less than orequal to 6, further preferably greater than or equal to 1 and less thanor equal to 6. Note that when z₂/y₂ is greater than or equal to 1 andless than or equal to 6, CAAC-OS films can be easily formed as the oxidesemiconductor films 107 and 115. Typical examples of the atomic ratio ofIn to M and Zn of the target are 1:2:4, 1.1:2.9:7.5, 1:3:2, 1:3:4,1:3:6, 1:3:8, 1:4:4, 1:4:5, 1:4:6, 1:4:7, 1:4:8, 1:5:5, 1:5:6, 1:5:7,1:5:8, and 1:6:8.

In each of the oxide semiconductor films 107, 111, and 115, theproportion of each atom in the above-described atomic ratio varieswithin a range of ±40% as an error.

The atomic ratio is not limited to the above, and may be appropriatelyset in accordance with needed semiconductor characteristics.

The oxide semiconductor film 107 and the oxide semiconductor film 115may have the same composition. For example, each of the oxidesemiconductor film 107 and the oxide semiconductor film 115 may be anIn—Ga—Zn oxide film with an atomic ratio of In:Ga:Zn=1:3:2, 1:3:4, or1:4:5.

Alternatively, the oxide semiconductor film 107 and the oxidesemiconductor film 115 may have different compositions. For example, theoxide semiconductor film 107 may be an In—Ga—Zn oxide film with anatomic ratio of In:Ga:Zn=1:3:2, and the oxide semiconductor film 115 maybe an In—Ga—Zn oxide film with an atomic ratio of In:Ga:Zn=1:3:4 or1:4:5.

The oxide semiconductor film 107 and the oxide semiconductor film 115may each have a thickness of more than or equal to 3 nm and less than orequal to 100 nm, or more than or equal to 3 nm and less than or equal to50 nm.

Here, it is preferable that the thickness of the oxide semiconductorfilm 111 be at least larger than that of the oxide semiconductor film107. The thicker the oxide semiconductor film 111 is, the larger theon-state current of the transistor can be. The thickness of the oxidesemiconductor film 107 may be set as appropriate as long as theformation of interface states at the interface with the oxidesemiconductor film 111 can be inhibited. For example, the thickness ofthe oxide semiconductor film 111 is larger than that of the oxidesemiconductor film 107, preferably 2 or more times, further preferably 4or more times, still further preferably 6 or more times as large as thatof the oxide semiconductor film 107. Note that the above descriptiondoes not apply in the case where the on-state current of the transistorneed not be increased, in which case the thickness of the oxidesemiconductor film 107 may be larger than or equal to that of the oxidesemiconductor film 111. In this case, more oxygen can be added to theoxide semiconductor film 107, and oxygen vacancies in the oxidesemiconductor film 111 can be reduced by heat treatment.

The thickness of the oxide semiconductor film 115 may be set asappropriate, in a manner similar to that of the oxide semiconductor film107, as long as the formation of interface states at the interface withthe oxide semiconductor film 111 can be inhibited. For example, thethickness of the oxide semiconductor film 115 may be set smaller than orequal to that of the oxide semiconductor film 107. If the oxidesemiconductor film 115 is thick, it may become difficult for theelectric field from the gate electrode 103 to reach the oxidesemiconductor film 111; thus, the oxide semiconductor film 115 ispreferably thin. In addition, to prevent oxygen contained in the oxidesemiconductor film 115 from diffusing to the pair of electrodes 113 aand 113 b and thus oxidizing the pair of electrodes 113 a and 113 b, theoxide semiconductor film 115 is preferably thin. For example, the oxidesemiconductor film 115 is preferably thinner than the oxidesemiconductor film 111. Note that the thickness of the oxidesemiconductor film 115 is not limited to the above, and may be set asappropriate in accordance with the driving voltage of the transistor inconsideration of the withstand voltage of the gate insulating film 117.

In the case where the oxide semiconductor film 107, the oxidesemiconductor film 111, and the oxide semiconductor film 115 havedifferent compositions from one another, the interfaces thereof can beobserved by scanning transmission electron microscopy (STEM) in somecases.

Hydrogen contained in the oxide semiconductor films 107, 111, and 115reacts with oxygen bonded to a metal atom to produce water, which causesoxygen vacancies in a lattice from which oxygen is released (or aportion from which oxygen is released). Electrons serving as carriersmight be generated when hydrogen enters the oxygen vacancies. Further,electrons serving as carriers might be generated when part of hydrogenis bonded to oxygen bonded to a metal atom. Thus, a transistor includingan oxide semiconductor which contains hydrogen is likely to be normallyon.

Accordingly, it is preferable that hydrogen be reduced as much aspossible as well as the oxygen vacancies in the oxide semiconductorfilms 107, 111, and 115. Specifically, the hydrogen concentration of theoxide semiconductor films 107, 111, and 115, which is measured by SIMS,is lower than or equal to 5×10¹⁹ atoms/cm³, lower than or equal to1×10¹⁹ atoms/cm³, lower than or equal to 5×10¹⁸ atoms/cm³, lower than orequal to 1×10¹⁸ atoms/cm³, lower than or equal to 5×10¹⁷ atoms/cm³, orlower than or equal to 1×10¹⁶ atoms/cm³. As a result, the transistor 100has a positive threshold voltage (normally-off characteristics).

Note that the impurity concentration in the oxide semiconductor films107, 111, and 115 can be measured by SIMS.

When the oxide semiconductor films 107, 111, and 115 contain silicon orcarbon that is a Group 14 element, oxygen vacancies in the filmsincrease to form n-type regions. Thus, the concentration of silicon orcarbon (the concentration is measured by SIMS) of each of the oxidesemiconductor films 107, 111, and 115 is set lower than or equal to2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³. Asa result, the transistor 100 has a positive threshold voltage(normally-off characteristics).

Furthermore, the concentration of alkali metal or alkaline earth metalof the oxide semiconductor films 107, 111, and 115, which is measured bySIMS, is set lower than or equal to 1×10¹⁸ atoms/cm³, preferably lowerthan or equal to 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metalmight generate carriers when bonded to an oxide semiconductor, which mayincrease the off-state current of the transistor. Thus, it is preferableto reduce the concentration of alkali metal or alkaline earth metal ofthe oxide semiconductor films 107, 111, and 115. As a result, thetransistor 100 has a positive threshold voltage (normally-offcharacteristics).

Further, when containing nitrogen, the oxide semiconductor films 107,111, and 115 easily become n-type by generation of electrons serving ascarriers and an increase of carrier density. Thus, a transistorincluding an oxide semiconductor film which contains nitrogen is likelyto be normally on. For this reason, nitrogen in the oxide semiconductorfilm is preferably reduced as much as possible; the concentration ofnitrogen which is measured by SIMS is preferably set, for example, lowerthan or equal to 5×10¹⁸ atoms/cm³.

When impurities in the oxide semiconductor films 107, 111, and 115 arereduced, the carrier density of the oxide semiconductor films can belowered. The oxide semiconductor films 107, 111, and 115 preferably havea carrier density of 1×10¹⁷/cm³ or less, further preferably 1×10¹⁵/cm³or less, still further preferably 1×10¹³/cm³ or less, yet furtherpreferably 1×10¹¹/cm³ or less.

When an oxide semiconductor film with a low impurity concentration and alow density of defect states is used as the oxide semiconductor films107, 111, and 115, the transistor can have more excellent electricalcharacteristics. Here, the state in which the impurity concentration islow and the density of defect states is low (the number of oxygenvacancies is small) is described as “highly purified intrinsic” or“substantially highly purified intrinsic.” A highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor has fewcarrier generation sources, and thus has a low carrier density in somecases. Thus, a transistor including the oxide semiconductor film inwhich a channel region is formed is likely to have positive thresholdvoltage (normally-off characteristics). A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has alow density of defect states and accordingly has few carrier traps insome cases. Further, a highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has an extremely lowoff-state current; the off-state current can be less than or equal tothe measurement limit of a semiconductor parameter analyzer, i.e., lessthan or equal to 1×10⁻¹³ A, at a voltage between a source electrode anda drain electrode (drain voltage) of from 1 V to 10 V. Thus, thetransistor whose channel region is formed in the oxide semiconductorfilm has a small variation in electrical characteristics and highreliability in some cases.

The oxide semiconductor films 107, 111, and 115 may have, for example, anon-single-crystal structure. The non-single-crystal structure includes,for example, a CAAC-OS which is described later, a polycrystallinestructure, a microcrystalline structure, or an amorphous structure.Among the non-single-crystal structure, the amorphous structure has thehighest density of defect states, whereas the CAAC-OS has the lowestdensity of defect states.

The oxide semiconductor films 107, 111, and 115 may have, for example, amicrocrystalline structure. The oxide semiconductor films 107, 111, and115 which have the microcrystalline structure each include amicrocrystal with a size, for example, greater than or equal to 1 nm andless than 10 nm. Alternatively, the oxide semiconductor films which havethe microcrystalline structure have, for example, a mixed phasestructure where crystal parts (each of which is greater than or equal to1 nm and less than 10 nm) are distributed in an amorphous phase.

The oxide semiconductor films 107, 111, and 115 may have, for example,an amorphous structure. The oxide semiconductor films 107, 111, and 115which have the amorphous structure each have, for example, disorderedatomic arrangement and no crystalline component. Alternatively, theoxide semiconductor films which have an amorphous structure have, forexample, an absolutely amorphous structure and no crystal part.

Note that the oxide semiconductor films 107, 111, and 115 may each be amixed film including regions having two or more of the followingstructures: a CAAC-OS, a microcrystalline structure, and an amorphousstructure. The mixed film, for example, has a single-layer structureincluding a region having an amorphous structure, a region having amicrocrystalline structure, and a region of a CAAC-OS. Alternatively,the mixed film may have, for example, a stacked-layer structureincluding a region having an amorphous structure, a region having amicrocrystalline structure, and a region of a CAAC-OS.

Note that the oxide semiconductor films 107, 111, and 115 may have, forexample, a single-crystal structure.

Oxygen vacancies in the oxide semiconductor film 111 can be reduced byproviding an oxide semiconductor film in which oxygen vacancies are lesslikely to be generated than in the oxide semiconductor film 111, overand under and in contact with the oxide semiconductor film 111. Further,since the oxide semiconductor film 111 is in contact with the oxidesemiconductor films 107 and 115 containing one or more metal elementsforming the oxide semiconductor film 111, the density of states at theinterface between the oxide semiconductor film 107 and the oxidesemiconductor film 111 and at the interface between the oxidesemiconductor film 111 and the oxide semiconductor film 115 is extremelylow. Thus, after oxygen is added to the oxide semiconductor film 107 or115, the oxygen is transferred from the oxide semiconductor film 107 or115 to the oxide semiconductor film 111 by heat treatment; however, theoxygen is hardly trapped by the interface states at this time, and theoxygen in the oxide semiconductor film 107 or 115 can be efficientlytransferred to the oxide semiconductor film 111. Accordingly, oxygenvacancies in the oxide semiconductor film 111 can be reduced. Sinceoxygen is added to the oxide semiconductor film 107 or 115, oxygenvacancies in the oxide semiconductor film 107 or 115 can be reduced. Inother words, the density of localized states of the oxide semiconductorfilm 111 and the oxide semiconductor film 107 or 115 can be reduced.

In addition, when the oxide semiconductor film 111 is in contact with aninsulating film including a different constituent element (e.g., a gateinsulating film including a silicon oxide film), an interface state issometimes formed and the interface state forms a channel. At this time,a second transistor having a different threshold voltage appears, sothat an apparent threshold voltage of the transistor is varied in somecases. However, since the oxide semiconductor films 107 and 115containing one or more kinds of metal elements forming the oxidesemiconductor film 111 are in contact with the oxide semiconductor film111, an interface state is not easily formed at the interfaces betweenthe oxide semiconductor film 107 and the oxide semiconductor film 111and between the oxide semiconductor film 115 and the oxide semiconductorfilm 111.

The oxide semiconductor films 107 and 115 function as barrier films thatprevent constituent elements of the gate insulating films 105 and 117from entering the oxide semiconductor film 111 and forming an impuritystate.

For example, in the case of using a silicon-containing insulating filmas the gate insulating films 105 and 117, silicon in the gate insulatingfilms 105 and 117 or carbon which might be contained in the gateinsulating films 105 and 117 enters the oxide semiconductor film 107 orthe oxide semiconductor film 115 to a depth of several nanometers fromthe interface in some cases. An impurity such as silicon or carbonentering the oxide semiconductor film 111 forms an impurity state. Theimpurity state serves as a donor to generate electrons; thus, the oxidesemiconductor film 111 might have n-type conductivity.

However, when the thicknesses of the oxide semiconductor film 107 andthe oxide semiconductor film 115 are each larger than severalnanometers, the impurity such as silicon or carbon does not reach theoxide semiconductor film 111, so that the influence of impurity state issuppressed.

Thus, providing the oxide semiconductor films 107 and 115 makes itpossible to reduce variation in the electrical characteristics of thetransistor, such as the threshold voltage.

In the case where a channel is formed at the interfaces between the gateinsulating film 105 and the oxide semiconductor film 111 and between thegate insulating film 117 and the oxide semiconductor film 111, interfacescattering occurs at the interfaces to reduce the field-effect mobilityof the transistor. However, since the oxide semiconductor films 107 and115 containing one or more kinds of metal elements forming the oxidesemiconductor film 111 are provided in contact with the oxidesemiconductor film 111, scattering of carriers does not easily occur atthe interfaces between the oxide semiconductor film 111 and each of theoxide semiconductor films 107 and 115, and thus the field-effectmobility of the transistor can be increased.

In this embodiment, the number of oxygen vacancies in the oxidesemiconductor film 111, and further the number of oxygen vacancies inthe oxide semiconductor film 107 in contact with the oxide semiconductorfilm 111 can be reduced; accordingly, the density of localized states ofthe oxide semiconductor film 111 can be reduced. As a result, thetransistor 100 in this embodiment has a small variation in thresholdvoltage and high reliability. Furthermore, the transistor 100 of thisembodiment has excellent electrical characteristics.

The pair of electrodes 113 a and 113 b is formed to have a single-layerstructure or a stacked-layer structure including, as a conductivematerial, any of metals such as aluminum, titanium, chromium, nickel,copper, yttrium, zirconium, molybdenum, silver, tantalum, manganese, andtungsten or an alloy containing any of these metals as a main component.Examples of the structure include a single-layer structure of analuminum film containing silicon, a single-layer structure of a copperfilm containing manganese, a two-layer structure in which a titaniumfilm is stacked over an aluminum film, a two-layer structure in which atitanium film is stacked over a tungsten film, a two-layer structure inwhich a copper film is stacked over a copper-magnesium alloy film, atwo-layer structure in which a copper film is stacked over acopper-magnesium-aluminum alloy film, a three-layer structure in which atitanium film or a titanium nitride film, an aluminum film or a copperfilm, and a titanium film or a titanium nitride film are stacked in thisorder, a three-layer structure in which a molybdenum film or amolybdenum nitride film, an aluminum film or a copper film, and amolybdenum film or a molybdenum nitride film are stacked in this order,and a three-layer structure in which a copper-magnesium alloy film, acopper film, and a copper-magnesium alloy film are stacked in thisorder. Note that a transparent conductive material containing indiumoxide, tin oxide, or zinc oxide may be used.

Note that in the oxide semiconductor film 111, a channel formationregion refers to a region that overlaps with the gate electrode 119 andis positioned between the pair of electrodes 113 a and 113 b. Further, achannel region refers to a region through which carriers mainly flow inthe channel formation region. Here, a channel region is part of theoxide semiconductor film 111, which is positioned between the pair ofelectrodes 113 a and 113 b. A channel length refers to the distancebetween the pair of electrodes 113 a and 113 b.

For the pair of electrodes 113 a and 113 b, it is preferable to use aconductive material that is easily bonded to oxygen, such as tungsten,titanium, aluminum, copper, molybdenum, chromium, or tantalum, or analloy thereof. Tungsten or titanium with a relatively high melting pointis preferably used, which allows subsequent process temperatures to berelatively high. Note that the conductive material that is easily bondedto oxygen includes a material to which oxygen is easily diffused. Inthis case, oxygen in the oxide semiconductor film 111 and the conductivematerial contained in the pair of electrodes 113 a and 113 b are bonded,and accordingly, an oxygen vacancy region is formed in the oxidesemiconductor film 111. Further, in some cases, part of constituentelements of the conductive material forming the pair of electrodes 113 aand 113 b is mixed into the oxide semiconductor film 111. In this case,n-type regions (low resistance regions) are formed in regions in contactwith the pair of electrodes 113 a and 113 b at least in the oxidesemiconductor film 111. The n-type regions (low resistance regions)function as a source region and a drain region.

A region having a high oxygen concentration may be formed in part of thepair of electrodes 113 a and 113 b in contact with the low-resistanceregions. Constituent elements of the oxide semiconductor film 111 enterthe pair of electrodes 113 a and 113 b in contact with thelow-resistance regions in some cases. In other words, in the vicinitiesof the interfaces between the oxide semiconductor film 111 and the pairof electrodes 113 a and 113 b, regions which can be called mixed regionsor mixed layers of the two contacting layers are formed in some cases.

Since the n-type regions (low-resistance regions) have highconductivity, the contact resistance between the oxide semiconductorfilm 111 and the pair of electrodes 113 a and 113 b can be reduced toincrease the on-state current of the transistor.

The gate insulating film 117 can be formed using any of the materialsthat can be used for the gate insulating film 105 as appropriate.

The gate electrode 119 can be formed using any of the materials that canbe used for the gate electrode 103 as appropriate.

The insulating films 121 and 123 can be formed using any of thematerials and formation methods that can be used for the gate insulatingfilm 117, as appropriate. Although a stacked-layer structure of theinsulating films 121 and 123 is used here, a single-layer structure maybe used as well.

As the insulating film 121 or the insulating film 123, an aluminum oxidefilm is preferably used. Since the aluminum oxide film functions as abarrier film against hydrogen, water, and oxygen, when it is used as theinsulating film 121 or 123, release of oxygen contained in the oxidesemiconductor film 111 and diffusion of water, hydrogen, and the likefrom the outside to the oxide semiconductor film 111 can be prevented.

In the case where the insulating film 121 and the insulating film 123are oxide insulating films, an oxide insulating film containing oxygenin excess of the stoichiometric composition may be used as one or bothof the insulating film 121 and the insulating film 123. In this way, theoxygen in the insulating film is transferred to the oxide semiconductorfilm, so that oxygen vacancies can be filled with the oxygen and furtherreduced.

As the oxide insulating film containing oxygen in excess of thestoichiometric composition, it is preferable to use an oxide insulatingfilm from which oxygen molecules with more than or equal to 1.0×10¹⁸molecules/cm³ are released in thermal desorption spectroscopy(hereinafter referred to as TDS) at a surface temperature of the oxideinsulating film of higher than or equal to 100° C. and lower than orequal to 700° C., or higher than or equal to 100° C. and lower than orequal to 500° C.

The oxide insulating film containing oxygen in excess of thestoichiometric composition has such a thickness as to allow oxygen to besupplied to the oxide semiconductor film 111. For example, the thicknesscan be more than or equal to 50 nm and less than or equal to 500 nm, ormore than or equal to 50 nm and less than or equal to 400 nm.

As one or both of the insulating films 121 and 123, a nitride insulatingfilm having a low content of hydrogen content may be provided. Thenitride insulating film is preferably as follows, for example: thenumber of hydrogen molecules released from the nitride insulating filmis less than 5.0×10²¹ molecules/cm³, preferably less than 3.0×10²¹molecules/cm³, and further preferably less than 1.0×10²¹ molecules/cm³when measured by TDS at a surface temperature of the nitride insulatingfilm of higher than or equal to 100° C. and lower or equal to 700° C.,preferably higher than or equal to 100° C. and lower than or equal to500° C.

The nitride insulating film has such a thickness as to prevent entry ofimpurities such as hydrogen and water from the outside. For example, thethickness can be more than or equal to 50 nm and less than or equal to200 nm, preferably more than or equal to 50 nm and less than or equal to150 nm, and further preferably more than or equal to 50 nm and less thanor equal to 100 nm.

As the gate insulating film 105 of the transistor, an oxide insulatingfilm containing a small amount of nitrogen oxide and having a lowdensity of defect states is used. FIG. 2 shows the amount of change inthe threshold voltage (ΔV_(th)) of the transistor with respect to stresstime between before and after a gate BT stress test in which positivevoltage or negative voltage is applied to the gate of the transistor. InFIG. 2, the horizontal axis represents the logarithm of stress time andthe vertical axis represents the amount of change in the thresholdvoltage. The stress test conditions can be as follows: the maximumtemperature is 150° C.; the maximum driving voltage is 3.3 V; and thestress is applied for a given period of time, e.g., longer than or equalto 50 hours or longer than or equal to 100 hours.

A measurement method of the gate BT stress test is described here.First, substrate temperature is kept constant at given temperature(hereinafter, referred to as stress temperature) to measure the initialV_(g)−Id characteristics of the transistor.

Next, while the substrate temperature is kept at stress temperature, thepair of electrodes serving as a source electrode and a drain electrodeof the transistor is set at the same potential and the gate electrode issupplied with a potential different from that of the pair of electrodesfor a certain period of time (hereinafter referred to as stress time).Then, the V_(g)−Id characteristics of the transistor are measured whilethe substrate temperature is kept at the stress temperature. As aresult, a difference in threshold voltage and a difference in shiftvalue between before and after the gate BT stress test can be obtainedas the amount of change in the electrical characteristics.

Note that a stress test where negative voltage is applied to a gateelectrode is called negative gate BT stress test (dark negative stress);whereas a stress test where positive voltage is applied is calledpositive gate BT stress test (dark positive stress). Note that a stresstest where negative voltage is applied to a gate electrode while lightemission is performed is called negative gate BT photostress test(negative photostress); whereas a stress test where positive voltage isapplied while light emission is performed is called positive gate BTphotostress test (positive photostress).

In FIG. 2, the solid line represents the amount of change in thresholdvoltage with respect to stress time. As shown in FIG. 2, the transistorof this embodiment has an amount of change in the threshold voltage withrespect to stress time of greater than −0.1 V and less than 0.1 V.

The transistor shown in this embodiment has a small amount of change inthreshold voltage over time and has high reliability.

An insulating film having a function of a base film of the oxidesemiconductor film is formed at a temperature higher than thetemperature of heat treatment performed on the oxide semiconductor film,whereby the nitrogen concentration of the insulating film can bereduced. This prevents production of nitrogen oxide and an increase indefects in the insulating film even when heat treatment is performed.

When the oxide semiconductor film 111 is formed after addition of oxygento the oxide semiconductor film 107, and then heat treatment isperformed on the oxide semiconductor film 111, oxygen contained in theoxide semiconductor film 107 is diffused into the oxide semiconductorfilm 111. As a result, oxygen vacancies in the oxide semiconductor film111 can be reduced.

<Manufacturing Method of Semiconductor Device>

A method for manufacturing a semiconductor device is described withreference to FIGS. 3A to 3D and FIGS. 4A to 4C.

Films of the transistor (e.g., an insulating film, an oxidesemiconductor film, a metal oxide film, and a conductive film) can beformed by a sputtering method, a chemical vapor deposition (CVD) method,a vacuum vapor deposition method, or a pulsed laser deposition (PLD)method. Alternatively, a coating method or a printing method can beused. Although the sputtering method and a plasma-enhanced chemicalvapor deposition (PECVD) method are typical examples of the filmformation method, a thermal CVD method may be used as well. As a thermalCVD method, a metal organic chemical vapor deposition (MOCVD) method oran atomic layer deposition (ALD) method may be used, for example.

Deposition by the thermal CVD method may be performed in such a mannerthat the pressure in a chamber is set to an atmospheric pressure or areduced pressure, and a source gas and an oxidizer are supplied to thechamber at a time and react with each other in the vicinity of thesubstrate or over the substrate. Thus, no plasma is generated in thedeposition; therefore, the thermal CVD method has an advantage that nodefect due to plasma damage is caused.

Deposition by the ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching the respective switching valves (also referredto as high-speed valves). In such a case, an inert gas (e.g., argon ornitrogen) or the like is introduced at the same time as or after theintroduction of a first source gas, and then a second source gas isintroduced so that the source gases are not mixed. Note that in the casewhere the first source gas and the inert gas are introduced at a time,the inert gas serves as a carrier gas, and the inert gas may also beintroduced at the same time as the introduction of the second sourcegas. Alternatively, the first source gas may be exhausted by vacuumevacuation instead of the introduction of the inert gas, and then thesecond source gas may be introduced. The first source gas is adsorbed onthe surface of the substrate to form a first layer, and then the secondsource gas is introduced to react with the first layer; as a result, asecond layer is stacked over the first layer, so that a thin film isformed.

The sequence of the gas introduction is repeated a plurality of timesuntil a desired thickness is obtained, whereby a thin film withexcellent step coverage can be formed. The thickness of the thin filmcan be adjusted by the number of repetition times of the gasintroduction sequence; therefore, the ALD method makes it possible toaccurately adjust the thickness and thus is suitable for manufacturing aminute FET.

As illustrated in FIG. 3A, the gate electrode 103 is formed over thesubstrate 101, an insulating film 104 is formed over the substrate 101and the gate electrode 103, and then an oxide semiconductor film 106 isformed over the insulating film 104. Next, oxygen 108 is added to theoxide semiconductor film 106.

Note that the insulating film 104 is processed into the gate insulatingfilm 105 later. The oxide semiconductor film 106 is processed into theoxide semiconductor film 107 later.

A method for forming the gate electrode 103 will be described below.First, a conductive film is formed by a sputtering method, a chemicalvapor deposition (CVD) method such as a metal organic chemical vapordeposition (MOCVD) method, a metal chemical vapor deposition method, anatomic layer deposition (ALD) method, or a plasma-enhanced chemicalvapor deposition (PECVD) method, an evaporation method, a pulsed laserdeposition (PLD) method, or the like. Then, a mask is formed over theconductive film by a lithography process. Then, part of the conductivefilm is etched using the mask to form the gate electrode 103. Afterthat, the mask is removed.

A tungsten film can be formed as the conductive film with a depositionapparatus employing an ALD method. In that case, a WF₆ gas and a B₂H₆gas are sequentially introduced more than once to form an initialtungsten film, and then a WF₆ gas and an H₂ gas are introduced at atime, so that a tungsten film is formed. Note that an SiH₄ gas may beused instead of a B₂H₆ gas.

Here, a 20-nm-thick tungsten film is formed as the conductive film by asputtering method. Then, a mask is formed over the conductive film by alithography step, and the conductive film is wet-etched using the mask,so that the gate electrode 103 is formed.

The insulating film 104 can be formed by a sputtering method, a chemicalvapor deposition (CVD) method such as a metal organic chemical vapordeposition (MOCVD) method, an atomic layer deposition (ALD) method, or aplasma-enhanced chemical vapor deposition (PECVD) method, a pulsed laserdeposition (PLD) method, a coating method, a printing method, or thelike.

In the case where the insulating film 104 is formed using a siliconoxide film or a silicon oxynitride film, a deposition gas containingsilicon and an oxidizing gas are preferably used as a source gas.Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride. As the oxidizing gas,oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide can be given asexamples.

In the case where the insulating film 104 is formed by a CVD methodusing, as the oxidizing gas, a nitrogen-containing gas, typicallydinitrogen monoxide, nitrogen dioxide, or the like, the depositiontemperature is set at higher than or equal to 450° C. and lower than thestrain point of the substrate, higher than or equal to 500° C. and lowerthan the strain point of the substrate, or higher than or equal to 500°C. and lower than or equal to 550° C. In that case, the amount ofnitrogen or nitrogen oxide contained in the insulating film 104 can bereduced. Hence, even when oxygen is added to the insulating film 104 ina subsequent step of adding oxygen to an oxide semiconductor film 106 b,the amount of nitrogen oxide produced in a later heat treatment step canbe reduced.

In the case where a gallium oxide film is formed as the insulating film104, an MOCVD method can be employed.

In the case where a hafnium oxide film is formed as the insulating film104 by a thermal CVD method such as an MOCVD method or an ALD method,two kinds of gases are used: ozone (O₃) as an oxidizer and a sourcematerial gas which is obtained by vaporizing a liquid containing asolvent and a hafnium precursor compound (a hafnium alkoxide solution,which is typified by tetrakis(dimethylamide)hafnium (TDMAH)). Note thatthe chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄.Examples of another material liquid includetetrakis(ethylmethylamide)hafnium.

In the case where an aluminum oxide film is formed as the insulatingfilm 104 by a thermal CVD method such as an MOCVD method or an ALDmethod, two kinds of gases are used: H₂O as an oxidizer and a sourcematerial gas which is obtained by vaporizing a liquid containing asolvent and an aluminum precursor compound (e.g., trimethylaluminum(TMA)). Note that the chemical formula of trimethylaluminum is Al(CH₃)₃.Examples of another material liquid include tris(dimethylamide)aluminum,triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

In the case where a silicon oxide film is formed as the insulating film104 by a thermal CVD method such as an MOCVD method or an ALD method,hexachlorodisilane is adsorbed on a deposition surface, chlorinecontained in adsorbate is removed, and radicals of an oxidizing gas(e.g., O₂ or dinitrogen monoxide) are supplied to react with theadsorbate.

Here, as the insulating film 104, a 100-nm-thick silicon oxynitride filmis formed by a CVD method; silane and dinitrogen monoxide are used asthe source gas and the deposition temperature is 500° C.

Then, heat treatment may be performed to release water, hydrogen, or thelike contained in the insulating film 104. This reduces theconcentration of water, hydrogen, or the like contained in the gateinsulating film 105 that is to be formed later. The heat treatment canreduce the amount of water, hydrogen, or the like diffused into theoxide semiconductor film 111.

The oxide semiconductor film 106 can be formed by a sputtering method, acoating method, a pulsed laser deposition method, a laser ablationmethod, a metal organic chemical vapor deposition (MOCVD) method, anatomic layer deposition (ALD) method, or the like.

In the case where the oxide semiconductor film 106 is formed by asputtering method, a power supply device for generating plasma can be anRF power supply device, an AC power supply device, a DC power supplydevice, or the like as appropriate.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof using the mixed gas of a rare gas and oxygen, the proportion ofoxygen is preferably higher than that of the rare gas.

Further, a target may be appropriately selected in accordance with thecomposition of the oxide semiconductor film 106 to be formed.

For example, in the case where the oxide semiconductor film is formed bya sputtering method at a substrate temperature higher than or equal to150° C. and lower than or equal to 750° C., preferably higher than orequal to 150° C. and lower than or equal to 450° C., further preferablyhigher than or equal to 200° C. and lower than or equal to 350° C., aCAAC-OS film can be formed as the oxide semiconductor film.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By inhibiting entry of impurities during the deposition, the crystalstate can be prevented from being broken by the impurities. For example,the concentration of impurities (e.g., hydrogen, water, carbon dioxide,or nitrogen) that exist in a deposition chamber may be reduced.Furthermore, the concentration of impurities in a deposition gas may bereduced. Specifically, a deposition gas whose dew point is −80° C. orlower, preferably −100° C. or lower is used.

In the case where an oxide semiconductor film, e.g., an InGaZnO_(X)(X>0) film is formed using a deposition apparatus employing an ALDmethod, an In(CH₃)₃ gas and an O₃ gas are sequentially introduced two ormore times to form an InO₂ layer, a Ga(CH₃)₃ gas and an O₃ gas areintroduced at a time to form a GaO layer, and then a Zn(CH₃)₂ gas and anO₃ gas are introduced at a time to form a ZnO layer. Note that the orderof these layers is not limited to this example. A mixed compound layersuch as an InGaO₂ layer, an InZnO₂ layer, or a GaZnO layer may be formedby mixing of these gases. Note that although an H₂O gas which is bubbledwith an inert gas such as Ar may be used instead of an O₃ gas, it ispreferable to use an O₃ gas, which does not contain H. Instead of anIn(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃ gas, aGa(C₂H₅)₃ gas may be used.

Here, as the oxide semiconductor film 106, a 20-nm-thick In—Ga—Zn oxidefilm (In:Ga:Zn=1:3:4) is formed by a sputtering method.

The oxygen 108 added to the oxide semiconductor film 106 is at least onekind selected from oxygen radicals, oxygen atoms, oxygen atomic ions,oxygen molecular ions, and the like. As a method for adding the oxygen108 to the oxide semiconductor film 106, an ion doping method, an ionimplantation method, and the like can be given.

In the case of using an ion implantation method as the method for addingthe oxygen 108, damage to the oxide semiconductor film 106 can bereduced by using oxygen molecular ions as the oxygen 108 added to theoxide semiconductor film 106. Oxygen molecular ions are broken down intooxygen atomic ions at the surface of the oxide semiconductor film 106,and the oxygen atomic ions are added to the oxide semiconductor film106. Since the energy for breaking oxygen molecules down into oxygenatoms is used, the energy per oxygen atomic ion in the case of addingoxygen molecular ions to the oxide semiconductor film 106 is lower thanthat in the case of adding oxygen atomic ions to the oxide semiconductorfilm 106. Therefore, in the case of adding oxygen molecular ions to theoxide semiconductor film 106, damage to the oxide semiconductor film 106can be reduced.

By using oxygen molecular ions, the energy of each oxygen atomic ioninjected to the insulating film 104 is lowered, which makes the injectedoxygen atomic ion be positioned in a shallow region. Accordingly, oxygenatoms easily move in later heat treatment, so that more oxygen can besupplied to an oxide semiconductor film 109 that is to be formed later.

In the case of injecting oxygen molecular ions, the energy per oxygenatomic ion is low as compared with the case of injecting oxygen atomicions. Thus, by using oxygen molecular ions for injection, theacceleration voltage can be increased and throughput can be increased.Moreover, by using oxygen molecular ions for injection, the dose can behalf of the amount that is necessary in the case of using oxygen atomicions. As a result, throughput can be increased.

In the case of adding oxygen to the oxide semiconductor film 106, it ispreferable that oxygen be added to the oxide semiconductor film 106 sothat a peak of the concentration profile of oxygen atomic ions can bepositioned in the oxide semiconductor film 106. As a result, damage tothe gate insulating film 105 that is to be formed later can be reduced.In other words, defects in the gate insulating film 105 can be reduced,so that variation in the electrical characteristics of the transistorcan be reduced. Furthermore, in the case where oxygen is added to theoxide semiconductor film 106 so that the amount of added oxygen atoms atthe interface between the insulating film 104 and the oxidesemiconductor film 106 is less than 1×10²¹ atoms/cm³, less than 1×10²⁰atoms/cm³, or less than 1×10¹⁹ atoms/cm³, the amount of oxygen added tothe gate insulating film 105 that is to be formed later can be reduced.As a result, damage to the gate insulating film 105 that is to be formedlater can be reduced, suppressing variation in the electricalcharacteristics of the transistor.

Even if oxygen is also added to the insulating film 104 in this step foradding oxygen, since the nitrogen concentration of the insulating film104 is low, the amount of nitrogen oxide produced by heat treatmentperformed later can be small and defects in a back channel of thetransistor can be reduced. Thus, there are a small number of carriertraps in the back channel of the transistor, leading to a reduction inthe amount of change in the threshold voltage of the transistor in a GBTstress test.

Oxygen may be added to the oxide semiconductor film 106 by plasmatreatment in which the oxide semiconductor film 106 is exposed to plasmagenerated in an atmosphere containing oxygen. As the oxygen-containingatmosphere, an atmosphere containing an oxidation gas such as oxygen,ozone, dinitrogen monoxide, or nitrogen dioxide can be given. Note thatit is preferable that the oxide semiconductor film 106 be exposed toplasma generated in a state where bias is applied on the substrate 101side, because the amount of oxygen added to the oxide semiconductor film106 can be increased. Examples of the apparatus used in such plasmatreatment include an ashing apparatus.

Here, oxygen atomic ions are added to the oxide semiconductor film 106by an ion implantation method at an acceleration voltage of 5 kV to adose of 1×10¹⁶/cm².

Through the above-described steps, an oxide semiconductor film 106 a towhich oxygen has been added can be formed as illustrated in FIG. 3B. Asa result, the number of oxygen vacancies in the oxide semiconductor film109 can be reduced in a later heat treatment step. Note that the oxidesemiconductor film 106 a to which oxygen has been added has a low filmdensity compared with the oxide semiconductor film 106 to which oxygenhas not been added.

Next, as illustrated in FIG. 3B, the oxide semiconductor film 109 isformed over the oxide semiconductor film 106 a to which oxygen has beenadded.

The oxide semiconductor film 109 can be formed by a sputtering method, acoating method, a pulsed laser deposition method, a laser ablationmethod, a metal organic chemical vapor deposition (MOCVD) method, anatomic layer deposition (ALD) method, or the like.

As a power supply device for generating plasma in forming the oxidesemiconductor film 109, an RF power supply device, an AC power supplydevice, a DC power supply device, or the like can be used asappropriate.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof using the mixed gas of a rare gas and oxygen, the proportion ofoxygen is preferably higher than that of the rare gas.

Further, a target may be appropriately selected in accordance with thecomposition of the oxide semiconductor film 109.

Note that in the case where the oxide semiconductor film 109 is formedby, for example, a sputtering method, the substrate temperature may beset to higher than or equal to 100° C. and lower than or equal to 450°C., preferably higher than or equal to 170° C. and lower than or equalto 350° C., and the oxide semiconductor film 109 may be formed whilebeing heated.

Here, as the oxide semiconductor film 109, a 20-nm-thick In—Ga—Zn oxidefilm (In:Ga:Zn=1:1:1) is formed by a sputtering method.

Next, heat treatment is performed so that part of oxygen contained inthe oxide semiconductor film 106 a to which oxygen has been added can betransferred to the oxide semiconductor film 109; consequently, oxygenvacancies in the oxide semiconductor film 109 can be reduced. This oxidesemiconductor film with reduced oxygen vacancies is represented as anoxide semiconductor film 109 a in FIG. 3C. Oxygen vacancies in the oxidesemiconductor film 106 a to which oxygen has been added can also bereduced. This oxide semiconductor film is represented as the oxidesemiconductor film 106 b in FIG. 3C. Hydrogen, water, and the likecontained in the oxide semiconductor film 106 a to which oxygen has beenadded and the oxide semiconductor film 109 can be released. This reducesthe amount of impurities contained in the oxide semiconductor film 106 ato which oxygen has been added and the oxide semiconductor film 109.

The temperature of heat treatment is preferably within the range oftemperatures at which oxygen moves to the oxide semiconductor film 109from the oxide semiconductor film 106 a to which oxygen has been added.In addition, the temperature of the heat treatment is preferably lowerthan the formation temperature of the insulating film 104. In that case,nitrogen oxide is unlikely to be produced in the insulating film 104 bythe heat treatment, so that the number of electron traps can be reduced.The temperature of the heat treatment is typically higher than or equalto 250° C. and lower than the strain point of the substrate, preferablyhigher than or equal to 300° C. and lower than or equal to 550° C.,further preferably higher than or equal to 350° C. and lower than orequal to 510° C., still further preferably higher than or equal to 350°C. and lower than or equal to 450° C.

The heat treatment is performed in an inert gas atmosphere containingnitrogen or a rare gas such as helium, neon, argon, xenon, or krypton.Further, after heat treatment is performed in an inert gas atmosphere,heat treatment may be additionally performed in an oxygen atmosphere ora dry air atmosphere (air whose dew point is lower than or equal to −80°C., preferably lower than or equal to −100° C., further preferably lowerthan or equal to −120° C.). Note that it is preferable that hydrogen,water, and the like be not contained in an inert gas and oxygen, likethe dry air, and the dew point is preferably lower than or equal to −80°C., further preferably lower than or equal to −100° C. The treatmenttime is 3 minutes to 24 hours.

In the heat treatment, instead of an electric furnace, any device forheating an object by heat conduction or heat radiation from a heatingelement such as a resistance heating element may be used. For example, arapid thermal anneal (RTA) apparatus such as a gas rapid thermal anneal(GRTA) apparatus or a lamp rapid thermal anneal (LRTA) apparatus can beused. An LRTA apparatus is an apparatus for heating an object byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high-temperature gas, an inert gas such asnitrogen or a rare gas like argon is used.

Here, after heat treatment is performed at 450° C. for 1 hour in anitrogen atmosphere, another heat treatment is performed at 450° C. for1 hour in an oxygen atmosphere.

Through the above-described steps, oxygen vacancies in the oxidesemiconductor films can be reduced. The oxide semiconductor films canhave a low density of localized states.

Note that the heat treatment may be performed in a later step, not inthis step. In other words, in another heating step performed later, partof oxygen contained in the oxide semiconductor film 106 a to whichoxygen has been added may be transferred to the oxide semiconductor film109. As a result, the number of heating steps can be reduced.

Then, after a mask is formed over the oxide semiconductor film 109 a bya lithography process, part of the oxide semiconductor film 106 b andpart of the oxide semiconductor film 109 a are etched using the mask.Thus, the oxide semiconductor film 107 and an oxide semiconductor film110 are formed as illustrated in FIG. 3D. Then, the mask is removed.Note that in the etching step, part of the insulating film 104 ispreferably etched. As a result, a transistor with the s-channelstructure where, in the channel width direction, the side surfaces ofthe oxide semiconductor film 107 and the oxide semiconductor film 111face the gate electrode 119 with the gate insulating film interposedtherebetween can be formed. Here, the insulating film 104 which ispartly etched is referred to as the gate insulating film 105.

Here, a mask is formed over the oxide semiconductor film 109 a by alithography process, and the oxide semiconductor film 106 b and theoxide semiconductor film 109 a are wet-etched using the mask, so thatthe oxide semiconductor film 107 and the oxide semiconductor film 110are formed.

Next, the pair of electrodes 113 a and 113 b is formed over the oxidesemiconductor film 110 as illustrated in FIG. 4A.

A method for forming the pair of electrodes 113 a and 113 b is describedbelow. A conductive film is formed by a sputtering method, a chemicalvapor deposition (CVD) method such as a metal organic chemical vapordeposition (MOCVD) method, a metal chemical vapor deposition method, anatomic layer deposition (ALD) method, or a plasma-enhanced chemicalvapor deposition (PECVD) method, an evaporation method, a pulsed laserdeposition (PLD) method, or the like. Then, a mask is formed over theconductive film by a lithography process. Then, part of the conductivefilm is etched using the mask to form the pair of electrodes 113 a and113 b. After that, the mask is removed.

Note that in the case where a transistor having an extremely shortchannel length is formed, at least the conductive film in a region todivide the pair of electrodes 113 a and 113 b is etched using a resistmask that is processed by a method suitable for micropatterning, such aselectron beam exposure, liquid immersion exposure, or EUV exposure. Notethat by the use of a positive type resist for the resist mask, theexposed region can be minimized to improve throughput. In theabove-described manner, a transistor having a channel length of 100 nmor less, further, 30 nm or less can be formed. Alternatively, minuteprocessing may be performed by an exposure technology which uses lightwith an extremely short wavelength (e.g., extreme ultraviolet (EUV)),X-rays, or the like.

Here, a 10 nm-thick tungsten film is formed as the conductive film by asputtering method. Then, a mask is formed over the conductive film by alithography process and the conductive film is dry-etched with use ofthe mask to form the pair of electrodes 113 a and 113 b.

After the pair of electrodes 113 a and 113 b is formed, cleaningtreatment is preferably performed to remove an etching residue. A shortcircuit of the pair of electrodes 113 a and 113 b can be suppressed bythis cleaning treatment. The cleaning treatment can be performed usingan alkaline solution such as a tetramethylammonium hydroxide (TMAH)solution, or an acidic solution such as diluted hydrofluoric acid, anoxalic acid solution, or a phosphoric acid solution. By the cleaningtreatment, part of the oxide semiconductor film 110 is etched, so thatthe oxide semiconductor film 111 having a depression is formed.

Next, as illustrated in FIG. 4B, the oxide semiconductor film 115 isformed over the oxide semiconductor film 111 and the pair of electrodes113 a and 113 b, the gate insulating film 117 is formed over the oxidesemiconductor film 115, and the gate electrode 119 is formed over thegate insulating film 117.

A method for forming the oxide semiconductor film 115, the gateinsulating film 117, and the gate electrode 119 is described below.First, an oxide semiconductor film is formed using any of the methodsthat can be used for forming the oxide semiconductor film 106. Next, aninsulating film is formed using any of the methods that can be used forforming the insulating film 104. Then, a conductive film is formed.Then, a mask is formed over the conductive film by a lithographyprocess. Next, the oxide semiconductor film, the insulating film, andthe conductive film are etched using the mask to form the oxidesemiconductor film 115, the gate insulating film 117, and the gateelectrode 119. After that, the mask is removed.

Here, as the oxide semiconductor film, a 5-nm-thick In—Ga—Zn oxide film(In:Ga:Zn=1:3:2) is formed by a sputtering method. Next, as theinsulating film, a 10-nm-thick silicon oxynitride film is formed by aCVD method. Then, as the conductive film, a 20-nm-thick tungsten film isformed by a sputtering method. Then, a mask is formed over theconductive film by a lithography process. Next, the oxide semiconductorfilm, the insulating film, and the conductive film are etched using themask to form the oxide semiconductor film 115, the gate insulating film117, and the gate electrode 119. After that, the mask is removed.

In the transistor 100, by providing the oxide semiconductor film 115 inwhich oxygen vacancies are unlikely to occur, release of oxygen fromside surfaces of the oxide semiconductor film 111 in the channel widthdirection is suppressed, so that generation of oxygen vacancies can besuppressed. As a result, a transistor that has improved electricalcharacteristics and high reliability can be provided.

Next, as illustrated in FIG. 4C, the insulating film 121 and theinsulating film 123 are formed sequentially over the gate insulatingfilm 105, the pair of electrodes 113 a and 113 b, the oxidesemiconductor film 115, the gate insulating film 117, and the gateelectrode 119. After that, heat treatment is preferably performed.

The insulating films 121 and 123 can be formed by a sputtering method, aCVD method, or the like as appropriate.

In the case where the insulating films 121 and 123 are oxide insulatingfilms containing oxygen in excess of the stoichiometric composition,they can be formed by a CVD method, a sputtering method, or the like.After the oxide insulating films are formed by a CVD method, asputtering method, or the like, oxygen may be added to the oxideinsulating films by an ion implantation method, an ion doping method,plasma treatment, or the like.

The heat treatment is preferably performed at a temperature lower thanthe formation temperature of the insulating film 104. In that case,nitrogen oxide is unlikely to be produced in the insulating film 104 bythe heat treatment. In addition, the number of carrier traps in the backchannel of the transistor can be reduced. The temperature of the heattreatment is typically higher than or equal to 150° C. and lower thanthe strain point of the substrate, preferably higher than or equal to250° C. and lower than or equal to 500° C., further preferably higherthan or equal to 350° C. and lower than or equal to 450° C.

Here, a 40-nm-thick aluminum oxide film is formed as the insulating film121 by a sputtering method, and a 150-nm-thick silicon oxynitride filmis formed as the insulating film 123 by a CVD method. Further, heattreatment is performed at 350° C. in an oxygen atmosphere for 1 hour.

Through the above-described steps, the density of localized states ofthe oxide semiconductor films is lowered, and thus a transistor withexcellent electrical characteristics can be manufactured. In addition, ahighly reliable transistor with a small variation in electricalcharacteristics with time or due to a stress test can be manufactured.

<Band Structure>

Here, a band structure is described. For easy understanding, the bandstructure is illustrated with the energy levels (Ec) at the conductionband minimum of the gate insulating film 105, the oxide semiconductorfilm 107, the oxide semiconductor film 111, the oxide semiconductor film115, and the gate insulating film 117.

As illustrated in FIGS. 5A and 5B, the energy at the conduction bandminimum changes continuously within the oxide semiconductor films 107,111, and 115. This can be understood also from the fact that theconstituent elements are common to the oxide semiconductor films 107,111, and 115 and oxygen is easily diffused among the oxide semiconductorfilms 107, 111, and 115. Thus, the oxide semiconductor films 107, 111,and 115 have a continuous physical property although they are a stack offilms having different compositions.

The oxide semiconductor films, which contain the same main componentsand are stacked, are not simply stacked but are formed to havecontinuous junction (here, in particular, a well structure having a Ushape in which the energy at the conduction band minimum continuouslychanges among the films (U-shape well)). In other words, a stacked-layerstructure is formed such that there exist no impurities which form adefect level such as a trap center or a recombination center at eachinterface. If impurities are mixed between the films in the stackedmultilayer film, the continuity of the energy band is lost and carriersdisappear by a trap or recombination at the interface.

Note that FIG. 5A shows the case where the energy level (Ec) at theconduction band minimum of the oxide semiconductor film 107 and that ofthe oxide semiconductor film 115 are equal to each other; however, theymay be different from each other. For example, FIG. 5B shows part of theband structure in the case where the energy level (Ec) at the conductionband minimum of the oxide semiconductor film 115 is closer to the vacuumlevel than that of the oxide semiconductor film 107 is.

As shown in FIGS. 5A and 5B, the oxide semiconductor film 111 serves asa well and a channel of the transistor 100 is formed in the oxidesemiconductor film 111. Note that a channel having a U-shaped wellstructure in which the energy at the conduction band minimumcontinuously changes like the one formed in the oxide semiconductorfilms 107, 111, and 115, can also be referred to as a buried channel.

Note that trap levels due to impurities or defects might be formed inthe vicinity of the interface between an insulating film such as asilicon oxide film and each of the oxide semiconductor films 107 and115. The oxide semiconductor film 111 can be distanced away from thetrap levels owing to the existence of the oxide semiconductor films 107and 115. However, when the difference between the energy (Ec) at theconduction band minimum of the oxide semiconductor film 107 or 115 andthe energy (Ec) at the conduction band minimum of the oxidesemiconductor film 111 is small, electrons in the oxide semiconductorfilm 111 might reach the trap levels across the energy difference. Whenelectrons to be negative charge are trapped in the trap levels, negativefixed charges are generated at the interface with the insulating film,whereby the threshold voltage of the transistor is shifted in thepositive direction.

Thus, to reduce a change in the threshold voltage of the transistor, theEc at the conduction band minimum of the oxide semiconductor film 111and the Ec at the conduction band minimum of each of the oxidesemiconductor films 107 and 115 needs to have an energy difference. Theenergy difference is preferably greater than or equal to 0.1 eV, furtherpreferably greater than or equal to 0.2 eV.

The oxide semiconductor films 107, 111, and 115 preferably include acrystal portion. In particular, a transistor including an oxidesemiconductor film having a c-axis aligned crystal can have stableelectrical characteristics.

In the band structure illustrated in FIG. 5B, instead of the oxidesemiconductor film 115, an In—Ga oxide film (e.g., an In—Ga oxide filmwith an atomic ratio of In:Ga=7:93) may be provided between the oxidesemiconductor film 111 and the gate insulating film 117.

For the oxide semiconductor film 111, an oxide semiconductor having anelectron affinity higher than those of the oxide semiconductor films 107and 115 is used. The oxide semiconductor used for the oxidesemiconductor film 111 has, for example, an electron affinity higherthan that of each of the oxide semiconductor films 107 and 115 by 0.07eV or higher and 1.3 eV or lower, preferably 0.1 eV or higher and 0.7 eVor lower, and further preferably 0.2 eV or higher and 0.4 eV or lower.

Since the transistor described in this embodiment includes the oxidesemiconductor films 107 and 115 each of which includes one or more kindsof metal elements included in the oxide semiconductor film 111,interface states are unlikely to occur at the interface between theoxide semiconductor film 107 and the oxide semiconductor film 111 andthe interface between the oxide semiconductor film 115 and the oxidesemiconductor film 111. Thus, providing the oxide semiconductor films107 and 115 makes it possible to reduce variation or change in theelectrical characteristics of the transistor, such as the thresholdvoltage.

When a channel is formed at the interface between the gate insulatingfilm 117 and the oxide semiconductor film 111, interface scatteringoccurs at the interface and the field-effect mobility of the transistormay decrease in some cases. In the transistor of this structure,however, the oxide semiconductor film 115 includes one or more kinds ofmetal elements included in the semiconductor film 111. Therefore,scattering of carriers is unlikely to occur at the interface between thesemiconductor film 111 and the oxide semiconductor film 115, and thusthe field-effect mobility of the transistor can be increased.

Modification Example 1

Transistors including an oxide semiconductor film and a gate insulatingfilm with different shapes from those of the oxide semiconductor film115 and the gate insulating film 117 in the transistor 100 illustratedin FIGS. 1A to 1C will be described with reference to FIGS. 6A to 6C.

A transistor 100 a illustrated in FIG. 6A includes an oxidesemiconductor film 115 a that is in contact with the oxide semiconductorfilm 111 and the pair of electrodes 113 a and 113 b, and a gateinsulating film 117 a that is in contact with the oxide semiconductorfilm 115 a. The gate insulating film 117 a is also in contact with thegate electrode 119.

In the transistor 100 a, end portions of the oxide semiconductor film115 a and the gate insulating film 117 a are located on the outer sideof an end portion of the gate electrode 119.

A transistor 100 b illustrated in FIG. 6B includes an oxidesemiconductor film 115 b that is in contact with the oxide semiconductorfilm 111 and the pair of electrodes 113 a and 113 b, and a gateinsulating film 117 b that is in contact with the oxide semiconductorfilm 115 b. The gate insulating film 117 b is also in contact with thegate electrode 119.

In the transistor 100 b, the oxide semiconductor film 115 b and the gateinsulating film 117 b are each not disconnected and cover the pair ofelectrodes 113 a and 113 b and the gate insulating film 105.

A transistor 100 c illustrated in FIG. 6C includes an oxidesemiconductor film 115 c that is in contact with the oxide semiconductorfilm 111 and the pair of electrodes 113 a and 113 b, and a gateinsulating film 117 b that is in contact with the oxide semiconductorfilm 115 c. The gate insulating film 117 b is also in contact with thegate electrode 119.

In the transistor 100 c, an end portion of the oxide semiconductor film115 c is located on the outer side of an end portion of the gateelectrode 119, and the gate insulating film 117 b is not disconnectedand covers the oxide semiconductor film 115 c, the pair of electrodes113 a and 113 b, and the gate insulating film 105.

The oxide semiconductor films 115 a, 115 b, and 115 c can be formedusing the same material as that of the oxide semiconductor film 115, asappropriate. The gate insulating films 117 a and 117 b can be formedusing the same material as that of the gate insulating film 117, asappropriate.

Here, manufacturing methods of the transistors 100 a, 100 b, and 100Cwill be described.

First, the manufacturing method of the transistor 100 a will bedescribed. Through the steps of FIGS. 3A to 3D and FIG. 4A, the gateelectrode 103, the gate insulating film 105, the oxide semiconductorfilm 107, the oxide semiconductor film 111, and the pair of electrodes113 a and 113 b are formed over the substrate 101.

Then, an oxide semiconductor film to be the oxide semiconductor film 115a is formed over the gate insulating film 105, the oxide semiconductorfilm 111, and the pair of electrodes 113 a and 113 b. Further, aninsulating film to be the gate insulating film 117 a is formed over theoxide semiconductor film to be the oxide semiconductor film 115 a. Next,a conductive film is formed over the insulating film to be the gateinsulating film 117 a. Further, a mask is formed over the conductivefilm by a lithography process, and etched using the mask to form thegate electrode 119. Then, the mask is removed.

In the etching step, since the pair of electrodes 113 a and 113 b iscovered with the insulating film to be the gate insulating film 117 a,surfaces of the pair of electrodes 113 a and 113 b can be prevented frombearing electrical charges. Consequently, electrostatic destructionbetween the gate electrode 119 and the pair of electrodes 113 a and 113b is unlikely to occur and yield can be increased.

Next, a mask is formed over the gate electrode 119 and the insulatingfilm to be the gate insulating film 117 a by a lithography process, andthe oxide semiconductor film to be the oxide semiconductor film 115 aand the insulating film to be the gate insulating film 117 a are etchedusing the mask, whereby the oxide semiconductor film 115 a and the gateinsulating film 117 a are formed.

In the etching step, since the gate electrode 119 is covered with themask, a surface of the gate electrode 119 can be prevented from bearingelectrical charges. Consequently, electrostatic destruction between thegate electrode 119 and the pair of electrodes 113 a and 113 b isunlikely to occur although the pair of electrodes 113 a and 113 b isexposed at the time of forming the oxide semiconductor film 115 a andthe gate insulating film 117 a; thus, yield can be increased.

The later steps are the same as those for the transistor 100 ofEmbodiment 1. Through the later steps, the transistor 100 a can bemanufactured.

As for the transistor 100 b, through the steps of FIGS. 3A to 3D andFIG. 4A, the gate electrode 103, the gate insulating film 105, the oxidesemiconductor film 107, the oxide semiconductor film 111, and the pairof electrodes 113 a and 113 b are formed over the substrate 101.

Next, the oxide semiconductor film 115 b is formed over the gateinsulating film 105, the oxide semiconductor film 111, and the pair ofelectrodes 113 a and 113 b, and the gate insulating film 117 b is formedover the oxide semiconductor film 115 b. Then, the gate electrode 119 isformed over the gate insulating film 117 b.

The later steps are the same as those for the transistor 100 ofEmbodiment 1. Through the later steps, the transistor 100 b can bemanufactured.

As for the transistor 100 c, through the steps of FIGS. 3A to 3D andFIG. 4A, the gate electrode 103, the gate insulating film 105, the oxidesemiconductor film 107, the oxide semiconductor film 111, and the pairof electrodes 113 a and 113 b are formed over the substrate 101.

Next, an oxide semiconductor film to be the oxide semiconductor film 115c is formed over the gate insulating film 105, the oxide semiconductorfilm 111, and the pair of electrodes 113 a and 113 b, and then a mask isformed over the oxide semiconductor film by a lithography process. Theoxide semiconductor film is etched using the mask to form the oxidesemiconductor film 115 c. Then, the mask is removed.

Next, the gate insulating film 117 b is formed over the oxidesemiconductor film 115 c. Then, the gate electrode 119 is formed overthe gate insulating film 117 b.

The later steps are the same as those for the transistor 100 ofEmbodiment 1. Through the later steps, the transistor 100 c can bemanufactured.

Modification Example 2

A transistor including a pair of electrodes with a different shape fromthat of the pair of electrodes 113 a and 113 b in the transistor 100illustrated in FIGS. 1A to 1C will be described with reference to FIGS.7A to 7D.

FIGS. 7A to 7D are a top view and cross-sectional views of a transistor100 d of a semiconductor device. FIG. 7A is a top view of the transistor100 d, FIG. 7B is a cross-sectional view taken along dashed-dotted lineA-B in FIG. 7A, FIG. 7C is a cross-sectional view taken alongdashed-dotted line C-D in FIG. 7A, and FIG. 7D is a cross-sectional viewtaken along dashed-dotted line E-F in FIG. 7A.

FIG. 7B is a cross-sectional view in the channel length direction of thetransistor 100 d, FIG. 7C is a cross-sectional view in the channel widthdirection of the transistor 100 d, and FIG. 7D is a cross-sectional viewin the channel width direction of the transistor 100 d showing theregion where the pair of electrodes and the oxide semiconductor filmsare stacked.

Note that in FIG. 7A, the substrate 101, the gate insulating film 105,the oxide semiconductor film 107, the gate insulating film 117, theinsulating film 121, the insulating film 123, and the like are notillustrated for simplicity.

The transistor 100 d illustrated in FIGS. 7A to 7D includes a pair ofelectrodes 113 c and 113 d that is not in contact with the side surfacesof the oxide semiconductor film 107 and the oxide semiconductor film 111and is in contact with the top surface of the oxide semiconductor film111. Furthermore, the transistor 100 d includes plugs 127 a and 127 bthat are in contact with the pair of electrodes 113 c and 113 d atopenings 125 a and 125 b provided in the insulating film 121 and theinsulating film 123.

As illustrated in FIG. 7D, since the side surfaces of the oxidesemiconductor films 107 and 111 are not in contact with the pair ofelectrodes 113 c and 113 d in the channel width direction, the electricfield of the gate electrode 119 is not blocked by the pair of electrodes113 c and 113 d. As a result, the effect of the electric field of thegate electrode 119 on the side surfaces of the oxide semiconductor films107 and 111 can be increased, and this transistor can therefore have anexcellent subthreshold swing (hereinafter referred to as S value) and ahigh field-effect mobility. Note that the S value is the value of a gatevoltage necessary to change the on-state current by one digit; a smallerS value means better transistor characteristics.

Next, a manufacturing method of the transistor 100 d will be describedwith reference to FIGS. 3A to 3D and FIGS. 8A to 8C. Here, themanufacturing method of the transistor 100 d will be described withreference to cross-sectional views taken along dashed-dotted lines A-Band C-D in FIG. 7A.

Through the steps of FIGS. 3A to 3C, the gate electrode 103, theinsulating film 104, the oxide semiconductor film 106 b, and the oxidesemiconductor film 109 a are formed over the substrate 101. Next, aconductive film 112 is formed over the oxide semiconductor film 109 a(see FIG. 8A).

The conductive film 112 can be formed by the formation method of thepair of electrodes 113 a and 113 b in Embodiment 1, as appropriate.

Next, a mask is formed over the conductive film 112 by a lithographyprocess, and then the oxide semiconductor film 106 b, the oxidesemiconductor film 109 a, and the conductive film 112 are etched usingthe mask to form the oxide semiconductor film 107, the oxidesemiconductor film 110, and a conductive film 113. In this step, part ofthe insulating film 104 is also etched, so that the gate insulating film105 is formed (see FIG. 8B).

The shape of the resist mask is changed in the etching step, so that ifetching is performed using only the resist mask without providing a hardmask, the shapes of the formed oxide semiconductor film 107 and oxidesemiconductor film 110 might be different from predetermined shapes.This problem arises outstandingly in micropatterning such as electronbeam exposure, liquid immersion exposure, or EUV exposure. However,since the conductive film 112 provided over the oxide semiconductor film109 a functions as a hard mask here, the oxide semiconductor film 107and the oxide semiconductor film 110 with predetermined shapes can beobtained.

Then, a mask is formed over the conductive film 113 by a lithographyprocess, and the conductive film 113 is etched using the mask to formthe pair of electrodes 113 c and 113 d. In the case of using apositive-type photoresist as the mask, the exposing time can beshortened. In the etching step, part of the oxide semiconductor film 110may be etched to form a depression. Here, the oxide semiconductor filmhaving a depression is referred to as the oxide semiconductor film 111(see FIG. 8C).

Then, by the steps similar to those of FIG. 4B, the oxide semiconductorfilm 115, the gate insulating film 117, and the gate electrode 119 areformed. In this way, the transistor 100 d illustrated in FIGS. 7A to 7Dcan be manufactured.

Note that as illustrated in FIG. 9, a pair of electrodes 113 g and 113 hthat is provided over the pair of electrodes 113 c and 113 d and is incontact with the side surfaces of the oxide semiconductor film 107 andthe oxide semiconductor film 111 in the channel length direction of thetransistor may be included. In FIG. 9, the electrode 113 g is in contactwith the electrode 113 c, and the electrode 113 h is in contact with theelectrode 113 d.

Modification Example 3

A transistor including the oxide semiconductor film 111 with a differentshape from that in the transistor 100 illustrated in FIGS. 1A to 1C willbe described with reference to FIGS. 10A to 10C.

FIGS. 10A to 10C are a top view and cross-sectional views of atransistor 100 e included in a semiconductor device. FIG. 10A is a topview of the transistor 100 e, FIG. 10B is a cross-sectional view takenalong dashed-dotted line A-B in FIG. 10A, and FIG. 10C is across-sectional view taken along dashed-dotted line C-D in FIG. 10A.

FIG. 10B is a cross-sectional view in the channel length direction ofthe transistor 100 e and FIG. 10C is a cross-sectional view in thechannel width direction of the transistor 100 e.

Note that in FIG. 10A, the substrate 101, the gate insulating film 105,the oxide semiconductor film 107, the gate insulating film 117, theinsulating film 121, the insulating film 123, and the like are notillustrated for simplicity.

As illustrated in FIG. 10C, the transistor 100 e includes an oxidesemiconductor film 111 a whose cross section in the channel widthdirection is substantially triangular or substantially trapezoidal.Here, the substantial triangle and the substantial trapezoid refer tothe shapes in which the angle between the bottom surface in contact withthe oxide semiconductor film 107 and the side surface in contact withthe oxide semiconductor film 115 in the oxide semiconductor film 111 ais greater than 0° and less than or equal to 85° or greater than orequal to 30° and less than or equal to 80°. In addition, the surface onthe opposite side to the bottom surface may have a sharp corner or arounded corner. The shape may have a vertex on the opposite side to thebottom surface.

As compared with an upper region in an oxide semiconductor film whosecross section in the channel width direction is substantiallyrectangular, an upper region in the oxide semiconductor film 111 a whosecross section is substantially triangular or trapezoidal has a smallcross-sectional area. This decreases a region having high currentdensity on the gate insulating film 117 side. As a result, excellent Svalue and increased on-state current can be achieved.

A method for manufacturing the oxide semiconductor film 111 with thesubstantially triangular or trapezoidal cross section will be described.A mask is formed over the oxide semiconductor film 109 a illustrated inFIG. 3C by a lithography process, and then, the oxide semiconductor film109 a is etched while the mask is made to recede. Thus, the oxidesemiconductor film whose cross section in the channel width direction issubstantially triangular or substantially trapezoidal as illustrated inFIG. 10C can be formed.

Modification Example 4

A transistor including the gate electrode with a different shape fromthat in the transistor 100 illustrated in FIGS. 1A to 1C will bedescribed with reference to FIGS. 11A to 11C.

FIGS. 11A to 11C are a top view and cross-sectional views of atransistor 100 j in a semiconductor device. FIG. 11A is a top view ofthe transistor 100 j, FIG. 11B is a cross-sectional view taken alongdashed-dotted line A-B in FIG. 11A, and FIG. 11C is a cross-sectionalview taken along dashed-dotted line C-D in FIG. 11A.

FIG. 11B is a cross-sectional view in the channel length direction ofthe transistor 100 j and FIG. 11C is a cross-sectional view in thechannel width direction of the transistor 100 j.

Note that in FIG. 11A, the substrate 101, the gate insulating film 105,the oxide semiconductor film 107, the gate insulating film 117, theinsulating film 121, the insulating film 123, and the like are notillustrated for simplicity.

As illustrated in FIGS. 11A and 11B, the transistor 100 j ischaracterized in that the pair of electrodes 113 a and 113 b does notoverlap with a gate electrode 119 a in the channel length direction. Asa result, parasitic capacitance between the pair of electrodes 113 a and113 b and the gate electrode 119 a can be reduced to increase theon-state current of the transistor.

After the gate electrode 119 a is formed, it is preferable to formimpurity regions 111 e and 111 f by adding impurities to the oxidesemiconductor film 111 using the gate electrode 119 a and the pair ofelectrodes 113 a and 113 b as masks. This increases the on-state currentof the transistor. Note that as the impurities added to the oxidesemiconductor film 111, hydrogen, helium, neon, argon, krypton, xenon,boron, nitrogen, phosphorus, or arsenic can be used.

Modification Example 5

A modification example of the transistor 100 illustrated in FIGS. 1A to1C will be described with reference to FIG. 43.

The transistor may include low-resistance regions 133 a and 133 bbetween the oxide semiconductor films 107 and 111 and the pair ofelectrodes 113 a and 113 b. The low-resistance regions 133 a and 133 bcan be formed using any of the light-transmitting conductive materialslisted as the materials of the gate electrode 103. The low-resistanceregions 133 a and 133 b can also be formed by adding hydrogen ornitrogen to the surfaces of the oxide semiconductor films 107 and 111.

The low-resistance regions 133 a and 133 b provided between the oxidesemiconductor films 107 and 111 and the pair of electrodes 113 a and 113b can reduce the contact resistance between the oxide semiconductorfilms 107 and 111 and the pair of electrodes 113 a and 113 b. As aresult, the on-state current of the transistor can be increased.

Modification Example 6

A modification example of the pair of electrodes included in thetransistor will be described with reference to FIGS. 44A and 44B. FIGS.44A and 44B are each an enlarged view of a region surrounded by a dashedline in FIG. 1B.

The pair of electrodes 113 a and 113 b preferably includes at least aCu—X alloy film (hereinafter, simply referred to as a Cu—X alloy film,where X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti). When the pair ofelectrodes is formed using, for example, a single-layer structure of aCu—X alloy film or a stacked-layer structure including a Cu—X alloyfilm, the resistance of the pair of electrodes can be reduced. Here, theelectrode 113 a is described as an example of the electrodes.

In the electrode 113 a illustrated in FIG. 44A, a Cu—X alloy film 134and a covering film 135 formed on a surface of the Cu—X alloy film 134are stacked. The covering film 135 contains X in the Cu—X alloy film anda compound formed by reaction between X and an element contained in theoxide semiconductor film 111 or 115 or the insulating film 121. Examplesof the compound containing X include an oxide containing X a nitridecontaining X silicide containing X and carbide containing X. Examples ofthe oxide containing X include an X oxide, an In—X oxide, a Ga—X oxide,an In—Ga—X oxide, and an In—Ga—Zn—X oxide. Since the covering film 135serves as a blocking film, entry of Cu in the Cu—X alloy film into theoxide semiconductor film 111 or 115 or the insulating film 121 can beprevented.

Note that when a Cu—Mn alloy film is used as an example of the Cu—Xalloy film 134, the adhesion between the electrode 113 a and the oxidesemiconductor film 111 or 115 or the insulating film 121 can beincreased. The use of the Cu—Mn alloy film results in a favorable ohmiccontact between the electrode 113 a and each of the oxide semiconductorfilms 111 and 115.

Specifically, the covering film 135 might be formed in the followingmanner: a Cu—Mn alloy film is formed as the Cu—X alloy film 134 and thenis subjected to heat treatment at higher than or equal to 150° C. andlower than or equal to 450° C., preferably higher than or equal to 250°C. and lower than or equal to 350° C., for example; and Mn in the Cu—Mnalloy film is segregated at the interfaces between the Cu—X alloy film134 and each of the oxide semiconductor films 111 and 115 and theinsulating film 121. The covering film 135 can contain a Mn oxide formedby oxidation of the segregated Mn, or an In—Mn oxide, a Ga—Mn oxide, anIn—Ga—Mn oxide, an In—Ga—Zn—Mn oxide, or the like, which is formed byreaction between the segregated Mn and a constituent element in theoxide semiconductor films 111 and 115. The covering film 135 increasesthe adhesion between the electrode 113 a and each of the oxidesemiconductor films 111 and 115. Furthermore, with the segregation of Mnin the Cu—Mn alloy film, part of the Cu—Mn alloy film becomes a pure Cufilm, so that the electrode 113 a can obtain high conductivity.

In the electrode 113 a illustrated in FIG. 44B, a conductive film 136, aCu—X alloy film 137, and a covering film 138 are stacked in this order.The conductive film 136 can be formed using a metal such as tungsten ortitanium or an alloy containing the metal. The materials for the Cu—Xalloy film 137 and the covering film 138 can be the same as those of theCu—X alloy film 134 and the covering film 135 illustrated in FIG. 44A.The conductive film 136 can prevent diffusion of Cu contained in theCu—X alloy film to the oxide semiconductor films 111 and 115.

The structures, methods, and the like described in this embodiment canbe used in appropriate combination with any of the structures, methods,and the like described in the other embodiments and examples.

Embodiment 2

Described in this embodiment is a method for reducing oxygen vacanciesin an oxide semiconductor film, which is different from the method inEmbodiment 1. This embodiment is different from Embodiment 1 in addingoxygen to an oxide semiconductor film positioned over the pair ofelectrodes 113 a and 113 b.

As illustrated in FIG. 12A, the gate electrode 103 is formed over thesubstrate 101, and the insulating film 104 is formed over the gateelectrode 103. Then, the oxide semiconductor film 106 is formed over theinsulating film 104, and the oxide semiconductor film 109 is formed overthe oxide semiconductor film 106.

Next, a mask is formed over the oxide semiconductor film 109 by alithography process, and part of the insulating film 104, part of theoxide semiconductor film 106, and part of the oxide semiconductor film109 are etched using the mask; thus, the gate insulating film 105, theoxide semiconductor film 107, and an oxide semiconductor film 110 a areformed as illustrated in FIG. 12B.

Next, heat treatment may be performed to release water, hydrogen, andthe like contained in the oxide semiconductor film 109. This heattreatment may be skipped and water, hydrogen, and the like contained inthe oxide semiconductor film 109 may be released by heat treatmentperformed in a later step.

Next, the pair of electrodes 113 a and 113 b is formed over the oxidesemiconductor film 110 a. After the pair of electrodes 113 a and 113 bis formed, cleaning treatment is preferably performed to remove anetching residue. A short circuit of the pair of electrodes 113 a and 113b can be suppressed by this cleaning treatment. By the cleaningtreatment, an oxide semiconductor film 111 b part of which is etched isformed (see FIG. 12C).

Next, as illustrated in FIG. 12D, an oxide semiconductor film 114 isformed over the gate insulating film 105, the oxide semiconductor film111 b, and the pair of electrodes 113 a and 113 b. Next, oxygen 108 isadded to the oxide semiconductor film 114.

As the oxygen 108 added to the oxide semiconductor film 114, at leastone kind selected from oxygen radicals, oxygen atoms, oxygen atomicions, oxygen molecular ions, and the like is used. As a method foradding the oxygen 108 to the oxide semiconductor film 114, an ion dopingmethod, an ion implantation method, or the like can be used.

In the case of adding oxygen to the oxide semiconductor film 114, it ispreferable that oxygen be added to the oxide semiconductor film 114 sothat a peak of the concentration profile of oxygen atomic ions ispositioned in the oxide semiconductor film 114. In the case where theoxide semiconductor film 114 is thin, oxygen might enter the oxidesemiconductor film 111 b; however, by using such conditions as to make apeak of the concentration profile of oxygen atomic ions be positioned inthe oxide semiconductor film 114, damage to the oxide semiconductor film111 b can be reduced. In other words, defects in the oxide semiconductorfilm 111 b can be reduced, so that variation in the electricalcharacteristics of the transistor can be reduced. Furthermore, in thecase where oxygen is added to the oxide semiconductor film 114 so thatthe amount of added oxygen atoms at the interface between the insulatingfilm 104 and the oxide semiconductor film 106 is less than 1×10²¹atoms/cm³, less than 1×10²⁰ atoms/cm³, or less than 1×10¹⁹ atoms/cm³,damage to the oxide semiconductor film 111 b can be reduced by laterheat treatment, whereby variation in the electrical characteristics ofthe transistor can be reduced.

In the case of using an ion implantation method as the method for addingthe oxygen 108, damage to the oxide semiconductor film 114 can bereduced by using oxygen molecular ions as the oxygen 108 added to theoxide semiconductor film 114.

Oxygen may be added to the oxide semiconductor film 114 by plasmatreatment in which the oxide semiconductor film 114 is exposed to plasmagenerated in an atmosphere containing oxygen.

Through the above-described steps, an oxide semiconductor film 114 a towhich oxygen has been added, which is illustrated in FIG. 13A, can beformed.

Next, heat treatment is performed so that part of oxygen contained inthe oxide semiconductor film 114 a to which oxygen has been added can betransferred to the oxide semiconductor film 111 b; consequently, oxygenvacancies in the oxide semiconductor film 111 b can be reduced. Thisoxide semiconductor film with reduced oxygen vacancies is represented asan oxide semiconductor film 111 c in FIG. 13B. Oxygen vacancies in theoxide semiconductor film 114 a to which oxygen has been added can bereduced. This oxide semiconductor film is represented as an oxidesemiconductor film 114 b in FIG. 13B.

Through the above-described steps, oxygen vacancies in the oxidesemiconductor films can be reduced. The oxide semiconductor films canhave a low density of localized states.

Then, part of the oxide semiconductor film 114 b is etched in a mannersimilar to that of Embodiment 1, so that an oxide semiconductor film 115d can be formed. Furthermore, the gate insulating film 117 and the gateelectrode 119 can be formed. Moreover, the insulating film 121 and theinsulating film 123 can be formed (see FIG. 13C).

Through the above-described steps, the density of localized states ofthe oxide semiconductor films is lowered, and thus a transistor withexcellent electrical characteristics can be manufactured. In addition, ahighly reliable transistor with a small variation in electricalcharacteristics with time or due to a stress test can be manufactured.

Modification Example 1

A method for adding oxygen to the oxide semiconductor film 114, which isdifferent from the above-described method in Embodiment 2, will bedescribed with reference to FIGS. 14A to 14C.

In a manner similar to that described in Embodiment 2, the gateelectrode 103, the gate insulating film 105, the oxide semiconductorfilm 107, the oxide semiconductor film 111 b, the pair of electrodes 113a and 113 b, and the oxide semiconductor film 114 are formed over thesubstrate 101. Next, the insulating film 116 is formed over the oxidesemiconductor film 114. Then, the oxygen 108 is added to the oxidesemiconductor film 114 through the insulating film 116 (see FIG. 14A).

Oxygen may be added to the oxide semiconductor film 114 through theinsulating film 116 by plasma treatment in which the insulating film 116is exposed to plasma generated in an atmosphere containing oxygen.

Through the above-described steps, the oxide semiconductor film 114 a towhich oxygen has been added and an insulating film 116 a to which oxygenhas been added, which are illustrated in FIG. 14B, can be formed.

Next, heat treatment is performed so that part of oxygen contained inthe oxide semiconductor film 114 a to which oxygen has been added andthe insulating film 116 a to which oxygen has been added can betransferred to the oxide semiconductor film 111 b; consequently, oxygenvacancies in the oxide semiconductor film 111 b can be reduced. Thisoxide semiconductor film with reduced oxygen vacancies is represented asthe oxide semiconductor film 111 c in FIG. 14C. Oxygen vacancies in theoxide semiconductor film 114 a to which oxygen has been added can bereduced. This oxide semiconductor film is represented as the oxidesemiconductor film 114 b in FIG. 14C. Defects in the insulating film 116a to which oxygen has been added can be reduced. This insulating film isrepresented as an insulating film 116 b in FIG. 14C.

Through the above-described steps, oxygen vacancies in the oxidesemiconductor films can be reduced. The oxide semiconductors film canhave a low density of localized states.

Then, part of the oxide semiconductor film 114 b is etched in a mannersimilar to that of Embodiment 1, so that the oxide semiconductor film115 a can be formed. In addition, part of the insulating film 116 b isetched, so that the gate insulating film 117 can be formed. Furthermore,the gate electrode 119 can be formed. Moreover, the insulating film 121and the insulating film 123 can be formed.

Through the above-described steps, the density of localized states ofthe oxide semiconductor films is lowered, and thus a transistor withexcellent electrical characteristics can be manufactured. In addition, ahighly reliable transistor with a small variation in electricalcharacteristics with time or due to a stress test can be manufactured.

The structures, methods, and the like described in this embodiment canbe used in appropriate combination with any of the structures, methods,and the like described in the other embodiments and examples.

Embodiment 3

Transistors having a stacked structure of oxide semiconductor filmsdifferent from that in the transistor 100 illustrated in FIGS. 1A to 1Cwill be described with reference to FIGS. 15A to 15C.

A transistor 100 f illustrated in FIG. 15A is different from thetransistor 100 illustrated in FIG. 1B in not including the oxidesemiconductor film 115. In other words, the transistor 100 f ischaracterized by including the gate insulating film 117 that is incontact with the oxide semiconductor film 111, the pair of electrodes113 a and 113 b, and the gate electrode 119.

Note that in a method for manufacturing the transistor 100 f illustratedin FIG. 15A, the method for forming the oxide semiconductor film 111described in Embodiment 1 can be used as appropriate.

A transistor 100 g illustrated in FIG. 15B is different from thetransistor 100 illustrated in FIG. 1B in not including the oxidesemiconductor film 107. In other words, the transistor 100 g ischaracterized by including the gate insulating film 105 that is incontact with the gate electrode 103 and the oxide semiconductor film111.

Note that in a method for manufacturing the transistor 100 g illustratedin FIG. 15B, the method for forming the oxide semiconductor film 111described in Embodiment 2 can be used as appropriate.

A transistor 100 h illustrated in FIG. 15C is different from thetransistor 100 illustrated in FIG. 1B in including an oxidesemiconductor film 115 e between the oxide semiconductor film 111 andthe pair of electrodes 113 a and 113 b. In other words, the transistor100 h is characterized by including the oxide semiconductor film 115 ethat is in contact with the oxide semiconductor film 111, the pair ofelectrodes 113 a and 113 b, and the gate insulating film 117. The oxidesemiconductor film 115 e is provided between the oxide semiconductorfilm 111 and the pair of electrodes 113 a and 113 b.

Note that in a method for manufacturing the transistor 100 h illustratedin FIG. 15C, the method for forming the oxide semiconductor film 111described in Embodiment 1 and/or Embodiment 2 can be used asappropriate.

The structures, methods, and the like described in this embodiment canbe used in appropriate combination with any of the structures, methods,and the like described in the other embodiments and examples.

Embodiment 4

Described in this embodiment are defects included in an oxidesemiconductor film in a transistor and an oxide insulating film incontact with the oxide semiconductor film, and the deterioration oftransistor characteristics.

<1. NO_(x)>

First, nitrogen oxide (hereinafter NO_(x); x is greater than or equal to0 and less than or equal to 2, preferably greater than or equal to 1 andless than or equal to 2) contained in the oxide insulating film incontact with the oxide semiconductor film is described.

<1-1. Transition Level of NO_(x) in Oxide Insulating Film>

First, transition levels of point defects in a solid are described. Atransition level shows the charge state of impurities or defects(hereinafter referred to as a defect D) forming a state in a gap, and iscalculated from the formation energy of defects. In other words, atransition level is similar to a donor level or an acceptor level.

The relationship between formation energy and transition levels of thecharge state of the defect D is described. The formation energy of thedefect D depends on the charge state and also depends on the Fermienergy. Note that D⁺ represents a state in which a defect releases oneelectron, D⁻ represents a state in which a defect traps one electron,and D⁰ represents a state in which no electron is transferred.

FIG. 16A illustrates the relationship between the formation energy andthe transition level of each of the defects D⁺, D⁰, and D⁻. FIG. 16Billustrates electron configurations of the defects D⁺, D⁰, and D⁻ in thecase where the defect D in a neutral state has an orbit occupied by oneelectron.

In FIG. 16A, the dotted line indicates the formation energy of thedefect D⁺, the solid line indicates the formation energy of the defectD⁰, and the dashed line indicates the formation energy of the defect D⁻.The transition level means the position of the Fermi level at which theformation energies of the defects D having different charge statesbecome equal to each other. The position of the Fermi level at which theformation energy of the defect D⁺ becomes equal to that of the defect D⁰(that is, a position at which the dotted line and the solid lineintersect) is denoted by ∈(+/0), and the position of the Fermi level atwhich the formation energy of the defect D⁰ becomes equal to that of thedefect D⁻ (that is, a position at which the solid line and the dashedline intersect) is denoted by ∈(0/−).

FIG. 17 is a conceptual diagram showing transition of charge states of adefect that are energetically stable when the Fermi level is changed. InFIG. 17, the dashed double-dotted line indicates the Fermi level. Rightviews of FIG. 17 are band diagrams of (1), (2), and (3) that indicatethe Fermi level in a left view of FIG. 17.

By finding out the transition level of a solid, it is qualitativelyknown that which charge state allows a defect to be energetically stableat each of the Fermi levels when the Fermi level is used as a parameter.

As a typical example of the oxide insulating film in contact with theoxide semiconductor film, a silicon oxynitride (SiON) film was used, andthe defect level in the silicon oxynitride film and an ESR signalattributed to the defect level were examined by calculation.Specifically, models in which NO₂, N₂O, NO, and an N atom wereintroduced into the respective silicon oxide (SiO₂) were formed, and thetransition levels thereof were examined to verify whether NO₂, N₂O, NO,and an N atom introduced into silicon oxide serve as electron traps ofthe transistor.

In calculation, SiO₂ (c-SiO₂) with a low-temperature quartz (α-quartz)crystal structure was used as a model. A crystal model of c-SiO₂ withoutdefects is shown in FIG. 18.

First, structure optimization calculation was performed on a modelincluding 72 atoms, particularly on the lattice constants and the atomiccoordinates. The model was obtained by doubling the unit cells in allaxis direction of c-SiO₂. In the calculation, first principlescalculation software VASP (the Vienna Ab initio Simulation Package) wasused. The effect of inner-shell electron was calculated by a projectoraugmented wave (PAW) method, and as a functional,Heyd-Scuseria-Ernzerhof (HSE) DFT hybrid factor (HSE06) was used. Thecalculation conditions are shown below.

TABLE 1 Software VASP Pseudopotential PAW method Functional HSE06 Mixingratio of exchange term 0.4 Cut-off energy 800 eV k-point 1 × 1 × 1(optimization) 2 × 2 × 2 (total energy)

The band gap of c-SiO₂ model after the structure optimization was 8.97eV that is close to the experimental value, 9.0 eV.

Next, the structure optimization calculation was performed on the abovec-SiO₂ models where NO₂, N₂O, NO, and an N atom were introduced intospaces (interstitial sites) in respective crystal structures. Thestructure optimization calculation was performed on each model withrespect to the following three cases: a case where the whole model ispositive monovalent (charge: +1); a case where the whole model iselectrically neutral (zerovalent) (charge: neutral); and a case wherethe whole model is negative monovalent (charge: −1). Note that thecharges imposed on the whole model, which were in the ground state ofelectrons, were localized in defects including NO₂, N₂O, NO, and an Natom.

As for the model in which NO₂ was introduced into an interstitial sitein the c-SiO₂ model, a structure after the structure optimizationcalculation was performed and structural parameters of an NO₂ are shownin FIG. 19. In FIG. 19, structural parameters of an NO₂ molecule in agaseous state are also shown as a reference example.

Note that the molecule that is not electrically neutral is frequentlycalled a molecular ion; however, unlike a gaseous state, it is difficultto quantitate the valence of molecule because the molecular discussedhere is one introduced inside a crystal lattice. Thus, a molecule thatis not electrically neutral is called molecular for convenience.

FIG. 19 shows that when an NO₂ molecule is introduced, the NO₂ moleculetends to be in a linear arrangement in the case where the charge of themodel is +1. FIG. 19 also shows that the angle of the O—N—O bond of themodel whose charge is −1 is smaller than that of the model whose chargeis neutral, and the angle of the O—N—O bond of the model whose charge isneutral is smaller than that of the model whose charge is +1. Thisstructure change in the NO₂ molecule is almost equal to a change in thebonding angle when the charge number of isolated molecules in a gasphase varies. Thus, it is suggested that almost the assumed charges areattributed to the NO₂ molecule, and the NO₂ molecule in SiO₂ probablyexists in a state close to an isolated molecule.

Next, as for the model in which an N₂O molecule was introduced into aninterstitial site in the c-SiO₂ model, a structure after the structureoptimization calculation was performed and structural parameters of theN₂O molecule are shown in FIG. 20. In FIG. 20, structural parameters ofthe N₂O molecule in a gaseous state are also shown as a referenceexample.

According to FIG. 20, in the case where the charge of the model is +1and the case where the charge is neutral, the structures of the N₂Omolecules are both in a linear arrangement, which means the N₂Omolecules of two cases have almost the same structure. In contrast, inthe case where the charge of the model is −1, the N₂O molecule has abent shape, and the distance between N and O is longer than that of theabove two cases. This conceivable reason is that an electron enters theLUMO level that is π* orbital of the N₂O molecule.

Next, as for the model in which an NO molecule was introduced into aninterstitial site in the c-SiO₂ model, a structure after the structureoptimization calculation was performed and structural parameters of theNO molecule are shown in FIG. 21.

According to FIG. 21, the distance between N and O is short in the casewhere the charge of the model is +1, and the distance between N and O islong in the case where the charge of the model is −1. This tendency isprobably caused by the following reason. In the case where the charge ofthe NO molecule in a gaseous state is +1, the bond order of the N—O bondis 3.0; in the case where the charge of the NO molecule in a gaseousstate is 0, the bond order is 2.5; and in the case where the charge ofthe NO molecule in a gaseous state is −1, the bond order is 2.0. Thus,the bond order is the largest when the charge is +1. Therefore, the NOmolecule in SiO₂ is considered to exist stably in a state close to theisolated molecule.

Then, as for the model in which an N atom was introduced into aninterstitial site in the c-SiO₂ model, a structure after the structureoptimization calculation was performed is shown in FIG. 22.

According to FIG. 22, in either charge state, the N atom that is bondedto atoms in SiO₂ is more stable in terms of energy than the N atom thatexists as an isolated atom in an interstitial site.

Next, the calculation of a transition level was performed on each model.

The transition level ∈(q/q′) for transition between the charge q stateand the charge q′ state in a model having defect D in its structure canbe calculated with Formula 1.

$\begin{matrix}{{{ɛ\left( {q/q^{\prime}} \right)} = \frac{{\Delta\; E^{q}} - {\Delta\; E^{q^{\prime}}}}{q^{\prime} - q}}{{\Delta\; E^{q}} = {{E_{tot}\left( D^{q} \right)} - {E_{tot}({bulk})} + {\sum\limits_{i}{n_{i}\mu_{i}}} + {q\left( {ɛ_{VBM} + {\Delta\; V_{q}} + E_{f}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the above formula, E_(tot) (D^(q)) represents the total energy in themodel having defect D of the charge q, E_(tot) (bulk) represents thetotal energy in a model without defects, represents the number of atomsi contributing to defects, represents the chemical potential of atom i,∈_(VBM) represents the energy of the valence band maximum in the modelwithout defects, ΔV_(q) represents the correction term relating to theelectrostatic potential, and E_(f) represents the Fermi energy.

FIG. 23 is a band diagram showing the transition levels obtained fromthe above formula. As the oxide semiconductor film, an In—Ga—Zn oxidefilm (hereinafter referred to as IGZO(111)) formed using metal oxidehaving an atomic ratio of In:Ga:Zn=1:1:1 is used. In FIG. 23, a banddiagram of the IGZO(111) is shown in addition to the band diagrams ofthe above four models. The unit of the values in FIG. 23 is “eV”.

In FIG. 23, the value of each transition level indicates a valueobtained when the valence band maximum of SiO₂ is considered as a base(0.0 eV). Although a reference value was used as an electron affinity ofSiO₂ here, the practical positional relation of the bands in the casewhere SiO₂ is bonded to the IGZO(111) is affected by the electronaffinity of SiO₂ in some cases.

Hereinafter, the transition level that transits between a state wherethe charge of the model is +1 and a state where the charge of the modelis 0 is referred to as (+/0), and the transition level that transitsbetween a state where the charge of the model is 0 and a state where thecharge of model is −1 is referred to as (0/−).

According to FIG. 23, in the model in which an NO₂ molecule wasintroduced into SiO₂, two transition levels of (+/0) and (0/−) exist atthe positions within the band gap of the IGZO(111), which suggests thatthe NO₂ molecule may relate to trap and detrap of electrons. In both amodel in which an NO molecule was introduced into SiO₂ and a model inwhich an N atom was introduced into SiO₂, the transition level of (+/0)exists at a position within the band gap of the IGZO(111). In contrast,the transition level of the model in which an N₂O molecule wasintroduced into SiO₂ exists outside of the band gap of the IGZO(111),and the N₂O molecules probably exist stably as neutral moleculesregardless of the position on the Fermi level.

The above results strongly suggest that interstitial moleculescontaining nitrogen, which relate to trap and detrap of electrons and isa factor causing a shift of the threshold voltage of a transistor in thepositive direction, have the transition level at a position closer tothe conduction band within the band gap of the IGZO(111). Here, amolecule having a transition level at a position closer to theconduction band in the band gap of the IGZO(111) is probably an NO₂molecule or an NO molecule, or both.

<1-2. Examination of ESR Signal>

Following the calculation results of the transition level, ESR signalsof NO₂ molecules were calculated. In addition, a model in which an Natom substituted for an O atom in SiO₂ was examined in a manner similarto that of the above case.

In this case, an N atom has seven electrons, and an O atom has eightelectrons; in other words, an electron structure of the NO₂ molecule hasan open shell. Thus, the neutral NO₂ molecule has a lone electron, andcan be measured by ESR. In the case where an N atom substitutes for an Oatom in SiO₂, only two Si atoms exist around an N atom and the N atomincludes a dangling bond. Thus, the case can also be measured by ESR.Furthermore, ¹⁴N has only one nuclear spin, and a peak of ESR signalrelating to ¹⁴N is split into three. At this time, the split width ofESR signal is a hyperfine coupling constant.

Thus, calculation was performed to examine whether split of an ESRsignal of the oxide insulating film into three is caused by the NO₂molecule or the N atom that replaces an O atom in SiO₂. When an SiO₂crystal structure is used as a model, the amount of calculation isenormous. Thus, in this case, two kinds of models of cluster structuresas shown in FIGS. 24A and 24B were used, the structure optimization wasperformed on these models, and then, g-factors and hyperfine couplingconstants were calculated. FIG. 24A shows a model of an NO₂ molecule ina neutral state, and FIG. 24B shows a cluster model including an Si—N—Sibond. Note that the model shown in FIG. 24B is a cluster model in whicha dangling bond of an Si atom is terminated with an H atom.

Amsterdam density functional (ADF) software was used for structureoptimization of the models and calculation of the g-factors andhyperfine coupling constants of the models whose structures wereoptimized. In the structure optimization of the models and thecalculation of the g-factors and hyperfine coupling constants of themodels whose structures were optimized, “GGA:BP” was used as afunctional, and “QZ4P” was used as a basic function, and “None” was usedas Core Type. In addition, in the calculation of the g-factors andhyperfine coupling constants, “Spin-Orbit” was considered as arelativistic effect, and as a calculation method of ESR/EPR, “g &A-Tensor (full SO)” was employed. The calculation conditions are asfollows.

TABLE 2 Software ADF Basis function QZ4P Functional GGA-BP Core TypeNone Relativistic Effect Spin-Orbit Calculation method of ESR/EPR g &A-Tensor (full SO)

As a result of structure optimization, in the case of the NO₂ moleculeshown in FIG. 24A, the bonding distance of the N—O bond was 0.1205 nm,and the angle of the O—N—O bond was 134.1°, which were close toexperimental values of the NO₂ molecule (the bonding distance: 0.1197nm, and the bonding angle 134.3°). In the case of the Si—N—Si clustermodel shown in FIG. 24B, the bonding distance of Si—N was 0.172 nm andthe angle of the Si—N—Si bond was 138.3°, which were almost the same asthe bonding distance of Si—N (0.170 nm) and the angle of the Si—N—Sibond)(139.0° in the structure that had been subjected to structureoptimization by first principles calculation in a state where an N atomsubstitutes for an O atom in the SiO₂ crystal.

The calculated g-factors and hyperfine coupling constants are shownbelow.

TABLE 3 g-factor Hyperfine coupling constant [mT] g_x g_y g_z g(average) A_x A_y A_z A (average) NO₂ 2.0066 1.9884 2.0014 1.9988 4.544.49 6.53 5.19 Si—N—Si 2.0021 2.0174 2.0056 2.0084 3.14 −0.61 −0.62 0.64

As described above, the hyperfine coupling constant A corresponds thedistance between the ESR signals (or the split width of a peak).According to Table 3, the average value of the hyperfine couplingconstant A of the NO₂ molecule is approximately 5 mT. In the case of theSi—N—Si cluster model, only A_x in the hyperfine coupling constants A isa positive value, which is approximately 3 mT. FIG. 45A and FIG. 45Bshow the ESR spectra of NO₂ and Si—N—Si, respectively, which arecalculated from the g-factor and the hyperfine coupling constant A.

According to this result, the ESR spectrum that has three signals, ahyperfine structure constant of approximately 5 mT, and a g-factor ofapproximately 2, which are obtained by ESR measurement using an X-band,is obtained probably because of an NO₂ molecule in an SiO₂ crystal.Among three signals, the g-factor of the medium signal is approximately2.

<1-3. Consideration of Deterioration Mechanism of Transistor>

A mechanism of a phenomenon in which the threshold voltage of atransistor is shifted in the positive direction when a positive gate BTstress test (+GBT) is performed is considered below based on the aboveresults.

The mechanism is considered with reference to FIG. 25. FIG. 25illustrates a structure in which a silicon oxynitride film (SiON film),an oxide semiconductor film (OS), a gate insulating film (GI), and agate (GE) are stacked in this order. Here, a case where the siliconoxynitride film SiON that is positioned on the back channel side of theoxide semiconductor film (OS) contains nitrogen oxide is described.

First, when the positive gate BT stress test (+GBT) is performed on thetransistor, the electron densities of the gate insulating film GI sideand the silicon oxynitride film SiON side of the oxide semiconductorfilm OS become higher. In the oxide semiconductor film OS, the siliconoxynitride film SiON side has a lower electron density than the gateinsulating film GI side. When an NO₂ molecule or an NO moleculecontained in the silicon oxynitride film SiON is diffused into theinterface between the gate insulating film GI and the oxidesemiconductor film OS and the interface between the oxide semiconductorfilm OS and the silicon oxynitride film SiON, electrons on the gateinsulating film GI side and the back channel side that are induced bythe positive gate BT stress test (+GBT) are trapped. As a result, thetrapped electrons remain in the vicinity of the interface between thegate insulating film GI and the oxide semiconductor film OS and theinterface between the oxide semiconductor film OS and the siliconoxynitride film SiON; thus, the threshold voltage of the transistor isshifted in the positive direction.

That is, a lower concentration of nitrogen oxide contained in thesilicon oxynitride film in contact with the oxide semiconductor film cansuppress a change in the threshold voltage of the transistor. Here, asspecific examples of the silicon oxynitride film in contact with theoxide semiconductor film, the protective film in contact with the backchannel side, the gate insulating film, and the like can be given. Byproviding the silicon oxynitride film containing an extremely smallamount of nitrogen oxide in contact with the oxide semiconductor film,the transistor can have excellent reliability.

Embodiment 5 Structure of Oxide Semiconductor

A structure of an oxide semiconductor will be described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

It is known that an amorphous structure is generally defined as beingmetastable and unfixed, and being isotropic and having no non-uniformstructure. In other words, an amorphous structure has a flexible bondangle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot beregarded as a completely amorphous oxide semiconductor. Moreover, anoxide semiconductor that is not isotropic (e.g., an oxide semiconductorthat has a periodic structure in a microscopic region) cannot beregarded as a completely amorphous oxide semiconductor. Note that ana-like OS has a periodic structure in a microscopic region, but at thesame time has a void and has an unstable structure. For this reason, ana-like OS has physical properties similar to those of an amorphous oxidesemiconductor.

<CAAC-OS>

First, a CAAC-OS will be described.

The CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

A CAAC-OS observed with TEM will be described below. FIG. 47A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 47B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 47A. FIG. 47B shows that metal atoms are arranged ina layered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which a CAAC-OS film is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS film, and is arranged parallel to theformation surface or the top surface of the CAAC-OS film.

As shown in FIG. 47B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 47C. FIGS. 47B and 47C prove that the size of apellet is greater than or equal to 1 nm or greater than or equal to 3nm, and the size of a space caused by tilt of the pellets isapproximately 0.8 nm. Therefore, the pellet can also be referred to as ananocrystal (nc). Furthermore, the CAAC-OS can also be referred to as anoxide semiconductor including c-axis aligned nanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 47D). The part in which the pellets are tilted as observed inFIG. 47C corresponds to a region 5161 shown in FIG. 47D.

FIG. 48A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 48B, 48C, and 48D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 48A,respectively. FIGS. 48B, 48C, and 48D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) will be described.For example, when the structure of a CAAC-OS including an InGaZnO₄crystal is analyzed by an out-of-plane method, a peak appears at adiffraction angle (2θ) of around 31° as shown in FIG. 49A. This peak isderived from the (009) plane of the InGaZnO₄ crystal, which indicatesthat crystals in the CAAC-OS have c-axis alignment, and that the c-axesare aligned in a direction substantially perpendicular to the formationsurface or the top surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 20 is around 36°, in addition tothe peak at 2 θ of around 31°. The peak at 2 θ of around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2 θ is around 31° and that apeak not appear when 2 θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray beam is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2 θ isaround 56°. This peak is attributed to the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (φ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (φ axis), as shown in FIG. 49B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when φ scan is performed with2θ fixed at around 56°, as shown in FIG. 49C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction will be described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 50A can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 50B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 50B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 50B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.The second ring in FIG. 50B is considered to be derived from the (110)plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with highcrystallinity. Entry of impurities, formation of defects, or the likemight decrease the crystallinity of an oxide semiconductor. This meansthat the CAAC-OS has small amounts of impurities and defects (e.g.,oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically; siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. For example,impurities contained in the oxide semiconductor might serve as carriertraps or carrier generation sources. Furthermore, oxygen vacancies inthe oxide semiconductor serve as carrier traps or serve as carriergeneration sources when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies isan oxide semiconductor with a low carrier density. Such an oxidesemiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.Thus, the CAAC-OS can be referred to as an oxide semiconductor havingstable characteristics.

<nc-OS>

Next, an nc-OS will be described.

An nc-OS has a region in which a crystal part is observed and a regionin which a crystal part is not clearly observed in a high-resolution TEMimage. In most cases, the size of a crystal part included in the nc-OSis greater than or equal to 1 nm and less than or equal to 10 nm, orgreater than or equal to 1 nm and less than or equal to 3 nm. Note thatan oxide semiconductor including a crystal part whose size is greaterthan 10 nm and less than or equal to 100 nm is sometimes referred to asa microcrystalline oxide semiconductor. In a high-resolution TEM imageof the nc-OS, for example, a grain boundary is not clearly observed insome cases. Note that there is a possibility that the origin of thenanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, acrystal part of the nc-OS may be referred to as a pellet in thefollowing description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not observed.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method. Forexample, when the nc-OS is analyzed by an out-of-plane method using anX-ray beam having a diameter larger than the size of a pellet, a peakwhich shows a crystal plane does not appear. Furthermore, a diffractionpattern like a halo pattern is observed when the nc-OS is subjected toelectron diffraction using an electron beam with a probe diameter (e.g.,50 nm or larger) that is larger than the size of a pellet. Meanwhile,spots appear in a nanobeam electron diffraction pattern of the nc-OSwhen an electron beam having a probe diameter close to or smaller thanthe size of a pellet is applied. Moreover, in a nanobeam electrondiffraction pattern of the nc-OS, bright regions in a circular (ring)pattern are shown in some cases. Also in a nanobeam electron diffractionpattern of the nc-OS, a plurality of spots are shown in a ring-likeregion in some cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<a-like OS>

An a-like OS has a structure intermediate between those of the nc-OS andthe amorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it includes a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (referred to as Sample A), an nc-OS (referred to as SampleB), and a CAAC-OS (referred to as Sample C) are prepared as samplessubjected to electron irradiation. Each of the samples is an In—Ga—Znoxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of an InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 51 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 51 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 51, acrystal part of approximately 1.2 nm (also referred to as an initialnucleus) at the start of TEM observation grows to a size ofapproximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². Incontrast, the crystal part size in the nc-OS and the CAAC-OS showslittle change from the start of electron irradiation to a cumulativeelectron dose of 4.2×10⁸ e⁻/nm. Specifically, as shown by (2) and (3) inFIG. 51, the average crystal sizes in an nc-OS and a CAAC-OS areapproximately 1.4 nm and approximately 2.1 nm, respectively, regardlessof the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit includes a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. It is difficult to deposit an oxidesemiconductor having a density of lower than 78% of the density of thesingle crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more films of an amorphous oxide semiconductor,an a-like OS, an nc-OS, and a CAAC-OS, for example.

The structures, methods, and the like described in this embodiment canbe used in appropriate combination with any of the structures, methods,and the like described in the other embodiments and examples.

Embodiment 6

In this embodiment, an example of a semiconductor device (memory device)that includes a transistor of one embodiment of the present invention,which can hold stored data even when not powered, and which has anunlimited number of write cycles, will be described with reference todrawings.

FIG. 26A is a cross-sectional view of a semiconductor device, and FIG.26B is a circuit diagram of a memory cell 760 included in thesemiconductor device.

The semiconductor device illustrated in FIGS. 26A and 26B includes atransistor 750 including a substrate 700 in a lower portion, and atransistor 200 including an oxide semiconductor and a capacitor 230 inan upper portion.

As the substrate 700, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate of silicon or silicon carbide, acompound semiconductor substrate of silicon germanium, asilicon-on-insulator (SOI) substrate, or the like may be used. Atransistor manufactured using a semiconductor substrate can operate athigh speed easily.

In this embodiment, an example in which an n-type single crystal siliconsubstrate is used as the substrate 700 is described. The transistor 750is a transistor whose channel is formed in the substrate 700. Thetransistor 750 includes a channel formation region 753, p-type impurityregions 754 functioning as lightly doped drain (LDD) regions orextension regions, p-type impurity regions 755 functioning as a sourceregion and a drain region, a gate insulating film 752, and a gateelectrode 751. The p-type impurity regions 755 have a higher impurityconcentration than the p-type impurity regions 754. A side surface ofthe gate electrode 751 is provided with a sidewall insulating film 756.With the use of the gate electrode 751 and the sidewall insulating film756 as masks, the p-type impurity regions 754 and the p-type impurityregions 755 can be formed in a self-aligned manner.

The transistor 750 is separated from other transistors 750 formed on thesubstrate 700 by an element separation region 789. Furthermore, aninsulating film 790 and an insulating film 791 are formed over andaround the gate electrode 751 and the sidewall insulating film 756.

The gate electrode 103, a conductive film 203, and a conductive film 204are formed over the insulating film 791. Note that the conductive film203 is connected to the gate electrode 751 of the transistor 750.

An insulating film 205 is formed between the gate electrode 103, theconductive film 203, and the conductive film 204. The insulating film205 can be formed using a material similar to that of the gateinsulating film 105 described in Embodiment 1 as appropriate.

An insulating film 206 that exposes part of the gate electrode 103, partof the conductive film 203, and part of the conductive film 204 isformed over the insulating film 205.

When an insulating film that has an effect of blocking water andhydrogen is used as the insulating film 206, water and hydrogencontained between the substrate 700 and the insulating film 206 can beprevented from diffusing into the oxide semiconductor film included inthe transistor 200. As the insulating film 206, a single layer or astacked layer including a material selected from aluminum oxide,aluminum oxynitride, silicon nitride, silicon nitride oxide, galliumoxide, germanium oxide, yttrium oxide, zirconium oxide, hafnium oxide,or tantalum oxide can be formed.

The gate insulating film 105 is formed over the gate electrode 103, theconductive film 203, the conductive film 204, and the insulating film206. The electrode 113 h, which is one of a pair of electrodes includedin the transistor 200, is electrically connected to the conductive film204 through an opening formed in the gate insulating film 105.

The transistor 200 that includes the gate electrode 103, the gateinsulating film 105, the pair of electrodes 113 g and 113 h, and thegate electrode 119 is formed over the insulating film 791. Any of thetransistors described in Embodiments 1 to 5 can be used as thetransistor 200. Here, the transistor illustrated in FIG. 9 is employedas the transistor 200.

The insulating film 121 is formed over the transistor 200 and theinsulating film 206. As the insulating film 121, the insulating film 121described in Embodiment 1 can be used as appropriate.

The insulating film 123 is formed over the insulating film 121. Theinsulating film 123 can be formed using a material and a method similarto those of the insulating film 123 described in Embodiment 1. The plug127 b is formed in an opening formed in the insulating film 123 and theinsulating film 121. The plug 127 b is electrically connected to theelectrode 113 h.

An insulating film 215 serving as a planarization insulating film isformed over the insulating film 123 and the plug 127 b. The insulatingfilm 215 can be formed of a heat-resistant organic material, such aspolyimide, acrylic, a benzocyclobutene-based resin, polyamide, or epoxy.Other than such organic materials, it is possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like. Note that the insulating film 215 may be formed by stacking aplurality of insulating films including these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include as a substituent anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.The organic group may include a fluoro group.

There is no particular limitation on the method for forming theinsulating film 215, and any of the following methods can be useddepending on the material: a sputtering method; an SOG method; spincoating; dipping; spray coating; a droplet discharging method (e.g., anink-jet method); a printing method (e.g., screen printing or offsetprinting); or the like. When the baking step of the insulating film 215also serves as heat treatment for another layer, a semiconductor devicecan be manufactured efficiently.

The insulating film 215 may be formed using a material and a methodsimilar to those of the insulating film 123 described in Embodiment 1and then may be subjected to CMP treatment.

A plug 216 is formed over the insulating film 215 and electricallyconnected to the plug 127 b through an opening formed in the insulatingfilm 215.

The gate electrode 751 is electrically connected to the conductive film203. One of the p-type impurity regions 755 in the transistor 750 iselectrically connected to a transistor 770 (see FIG. 26B), and the otherthereof is electrically connected to a wiring SL (see FIG. 26B).Furthermore, the electrode 113 h is electrically connected to a wiringBL (see FIG. 26B), the electrode 113 g is electrically connected to anode FN (see FIG. 26B), the gate electrode 119 is electrically connectedto a wiring WWL (see FIG. 26B), and the gate electrode 103 iselectrically connected to a wiring BGL (see FIG. 26B).

Here, a semiconductor material of region in which a channel region ofthe transistor 750 is formed and a semiconductor material of a region inwhich a channel region of the transistor 200 is formed preferably havedifferent band gaps. For example, in the case where an oxidesemiconductor is used as the semiconductor material of the region inwhich the channel region of the transistor 200 is formed, it ispreferable that a semiconductor material other than an oxidesemiconductor be used as the semiconductor material of the region inwhich the channel region of the transistor 750 is formed. A transistorincluding a semiconductor material other than an oxide semiconductor,such as crystalline silicon, can operate at high speed more easily thana transistor including an oxide semiconductor. A transistor including anoxide semiconductor enables charges to be held for a long time owing toits electrical characteristics, that is, the low off-state current.

A transistor including crystalline silicon as a semiconductor materialof a region in which a channel region of the transistor is formed canoperate at higher speed than a transistor including an oxidesemiconductor as a semiconductor material of a region in which a channelregion is formed. By using this transistor as a reading transistor, datacan be read at high speed.

Although both of the above transistors are p-channel transistors in thefollowing description, it is needless to say that n-channel transistorscan be used. Note that unless otherwise specified, the specificstructure of the semiconductor device, such as a material used for thesemiconductor device and the structure of the semiconductor device, neednot be limited to that described here.

The transistor 200 is a transistor that includes an oxide semiconductoras a semiconductor material in a region in which a channel region isformed. Since the off-state current of the transistor 200 is low, storeddata can be retained for a long period. In other words, powerconsumption can be sufficiently reduced because a semiconductor memorydevice in which refresh operation is unnecessary or the frequency ofrefresh operation is extremely low can be provided.

The transistor 200 and the capacitor 230 can be formed over thesubstrate on which the transistor 750 is formed as illustrated in FIG.26A, which enables a higher degree of integration of the semiconductordevice.

FIG. 26B is a circuit diagram of the memory cell 760. The memory cell760 includes the transistor 200, the transistor 750, the capacitor 230,and the transistor 770.

The transistor 200 includes an oxide semiconductor film in its channelformation region. Thus, the transistor 200 has an extremely lowoff-state current (extremely high off-state resistance). The transistor200 is turned on when data is written, and accordingly is also referredto as a writing transistor. The transistor 200 may be an n-channel orp-channel transistor. The following description is made on the casewhere the transistor 200 is an n-channel transistor.

The transistor 200 has a dual-gate structure, and one of the gates iselectrically connected to the wiring WWL. The wiring WWL can function asa write word line. The other gate is connected to the wiring BGL. Notethat the other gate may be always kept at a constant potential.

One of a source and a drain of the transistor 200 is electricallyconnected to the wiring BL. The wiring BL can function as a bit line.

The other of the source and the drain of the transistor 200 iselectrically connected to one electrode of the capacitor 230. The otherelectrode of the capacitor 230 is electrically connected to a wiring CL.The other of the source and the drain of the transistor 200 iselectrically connected to a gate of the transistor 750.

By changing the potential of the wiring CL, the potential of the gate ofthe transistor 750 (the node FN) is changed. The wiring CL is alsoreferred to as a capacitor line.

The transistor 750 is a p-channel transistor. Various materials such asan oxide semiconductor and silicon can be used for a channel formationregion of the transistor 750. One of a source and a drain of thetransistor 750 is electrically connected to one of a source and a drainof the transistor 770. The other of the source and the drain of thetransistor 750 is electrically connected to the wiring SL.

The other of the source and the drain of the transistor 770 iselectrically connected to the wiring BL. A gate of the transistor 770 iselectrically connected to a wiring RWL. The transistor 770 is atransistor for electrically connecting the transistor 750 and the wiringBL when data is read, and is also referred to as a selection transistor.

The wiring SL can function as a source line or a power supply line.Although the wiring SL is preferably retained at a constant potential,the potential may be changed when power is stopped or supplied.

In the memory cell 760 in FIG. 26B, data is retained as the potential ofthe node FN. If the transistor 200 has a sufficiently high off-stateresistance, data can be retained for a very long period. Theoretically,a data retention period is determined by all capacitance between thenode FN and each of the other nodes (including the capacitance of thecapacitor 230) and all resistance between the node FN and each of theother nodes (including the off-state resistance of the transistor 200).

For example, when the capacitance is 30 fF and the resistance is1×10²²Ω, the time constant is 9.5 years. Accordingly, after ten years,the difference between the potential of the node FN and a referencepotential is lowered to approximately 35% of the difference between theinitial potential and the reference potential. Data needs to be readcorrectly even in such a case where the potential is lowered.

Operation for writing data to the memory cell 760 and operation forreading data from the memory cell 760 are described below with referenceto FIG. 27. The threshold values of the transistors 750 and 770 are eachlower than 0 and higher than −VDD.

<Writing Operation>

To write data, the potential of the wiring BL, a bit line, is set to thepotential corresponding to the data when the transistor 200 is on. Thismethod is basically the same as the method for data writing to a DRAM.The transistor 200 is different from the transistors 750 and 770 in thethreshold value or the like; thus, here, when the transistor 200 isturned on, the potential of its gate (the potential of the wiring WWL)is set to V_(OS) _(_) _(H), and when the transistor 200 is turned off,the potential of its gate is set to V_(OS) _(_) _(L). Note that V_(OS)_(_) _(L) may be equal to GND (<VDD).

Here, when data “0” (one of two levels) is written, the potential of thewiring BL is set to GND, and when data “1” (the other of the two levels)is written, the potential of the wiring BL is set to VDD. At Time T1 inFIG. 27, the potential of the wiring WWL starts to increase, so that thetransistor 200 is turned on. As a result, the node FN has a potentialcorresponding to data. For example, when data “0” is written, thepotential of the node FN becomes GND, and when data “1” is written, thepotential of the node FN becomes VDD. At Time T2, the potential of thewiring WWL starts to decrease, so that the transistor 200 is turned off,and writing is finished. When the transistor 200 is turned off, thepotential of the node FN is slightly lowered by capacitive couplingbetween the gate of the transistor 200 (and the wiring WWL) and the nodeFN.

In writing, it is preferable that no current flow between the wirings BLand SL. For example, the wirings BL and SL may have no potentialdifference. That is, as in the case of the wiring BL, the potential ofthe wiring SL may be changed in accordance with data.

In a more effective method, the potential of the wiring RWL is set to apotential at which the transistor 770 is turned off. The potentials ofthe wirings BL and SL are each higher than or equal to GND and lowerthan or equal to VDD. Accordingly, when the potential of the wiring RWLis set to VDD, the transistor 770 is turned off. The potential of thewiring SL remains VDD except in the standby period in this embodiment,but may be another potential.

<Retention Operation>

When data is retained, the transistor 200 is turned off. In FIG. 27, ina period from Time T3 to Time T4, data is retained with power supplystopped (standby period). In the standby period, all the wirings havethe same potential (here, GND). In the case where the potential of thenode FN is higher than GND at this time, the potential of the node FN isgradually lowered.

In the case where data “0” has been written, the potential of the nodeFN is close to GND; thus, its change does not become a significantproblem. In contrast, in the case where data “1” has been written, thepotential of the node FN is close to VDD at first, but is lowered overtime. The potential decrease is ΔV. That is, the potential of the nodeFN (the potential of the gate of the transistor 750) is (VDD−ΔV) afterthe data retention period. Under the above conditions, the potential islowered by approximately 10% in the case where the retention period isabout one year, but is lowered to 35% of the initial potential in tenyears as described above. In other words, ΔV=0.65×VDD. Here, when thepotential decrease of the node FN is the largest in the period in whichdata retention is ensured, the potential of the node FN becomes(VDD−ΔV_(MAX)).

<Reading Operation>

To read data, the potentials of the wiring BL and the wiring SL are setto different potentials, the transistor 770 is turned on, and it isdetermined whether current flows between the source and the drain of thetransistor 750. The conduction state of the transistor 750 varies withthe potential of the node FN, whereby the data that has been written canbe determined.

Specifically, the potential of the wiring RWL is set to an appropriatevalue (here, VDD), the transistor 770 is turned off, and the potentialof the wiring SL is set to VDD. After the wiring BL is precharged to anappropriate potential (here, GND), the wiring BL is brought into afloating state. Then, the potential of the wiring CL is set to anappropriate value (here, a, note that GND<α<VDD).

In the case where data “0” has been written, the potential of the nodeFN is close to GND right before this time. However, owing to theincrease of the potential of the wiring CL from GND to a, the potentialof the node FN becomes almost a because of capacitive coupling throughthe capacitor 230. In the case where data “1” has been written, thepotential of the node FN becomes almost (VDD−ΔV+α−GND). After that, thepotential of the wiring RWL is set to an appropriate value (here, GND)at T5 to turn on the transistor 770.

In order that data is read correctly, in the case where data “0” hasbeen written, it is required that the transistor 750 is on and thepotential of the wiring BL increases from GND to VDD; in the case wheredata “1” has been written, it is required that the transistor 750 is offand the potential of the wiring BL remains GND.

To fulfill the requirements, the following two inequalities need to besatisfied when the threshold value of the transistor 750 is Vth:α<VDD+Vth and VDD−ΔV+α−GND VDD+Vth. That is,GND+ΔV+Vth≦GND+ΔV_(MAX)+Vth≦α<VDD+Vth.

For example, 0.7 [V]≦α<1.3 [V] is satisfied when VDD=+1.8 [V], GND=0[V], Vth=−0.5 [V], and ΔV_(MAX)=1.2 [V]. Alternatively, 0.2 [V]≦α<0.5[V] is satisfied when VDD=+0.9 [V], GND=0 [V], Vth=−0.4 [V], andΔV_(MAX)=0.6 [V].

Note that α can be a given value in the required range, and a may be theaverage of VDD and GND (also referred to as VDD/2) or the sum of GND andthe difference between VDD and GND that is divided by N (also referredto as VDD/N; note that N=3, 4, 5, . . . ). In the former case, VDD/2 is0.9 [V]. In the latter case, VDD/3 is 0.3 [V]. Both values are in therequired range.

As described above, in the case where the potential of the node FN islowered from the initial potential by greater than or equal to 60% (inthe case where the potential of the node FN is lowered to less than orequal to 40% of the initial potential) in the standby period, thepotential of the node FN is preferably increased by increasing thepotential of the wiring CL as appropriate in reading.

Note that although the written potential is initially VDD in the case ofdata “1”, the potential output to the wiring CL is GND. It needs to benoted that inverted data is output like this.

When including a transistor having a channel formation region formedusing an oxide semiconductor and having an extremely low off-statecurrent, the semiconductor device described in this embodiment canretain stored data for an extremely long time. In other words, refreshoperation becomes unnecessary or the frequency of the refresh operationcan be extremely low, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be retained for a long time evenwhen power is not supplied (note that a potential is preferably fixed).

Furthermore, the semiconductor device described in this embodiment doesnot need a high voltage for writing data and has no problem ofdeterioration of elements. Unlike in a conventional nonvolatile memory,for example, it is not necessary to inject and extract electrons intoand from a floating gate; thus, a problem such as deterioration of agate insulating film does not occur. That is, the semiconductor deviceof the disclosed invention does not have a limit on the number of timesdata can be rewritten, which is a problem of a conventional nonvolatilememory, and the reliability thereof is drastically improved.Furthermore, data writing depends on the state of the transistor (on oroff), whereby high-speed operation can be easily achieved.

As described above, a miniaturized and highly integrated semiconductordevice having high electrical characteristics can be provided.

The structures, methods, and the like described in this embodiment canbe used in appropriate combination with any of the structures, methods,and the like described in the other embodiments and examples.

Embodiment 7

In this embodiment, a structure example of a display device of oneembodiment of the present invention will be described.

Structure Example

FIG. 28A is a top view of the display device of one embodiment of thepresent invention. FIG. 28B is a circuit diagram illustrating a pixelcircuit that can be used in the case where a liquid crystal element isused in a pixel in the display device of one embodiment of the presentinvention. FIG. 28C is a circuit diagram illustrating a pixel circuitthat can be used in the case where an organic EL element is used in apixel in the display device of one embodiment of the present invention.

Transistors in a pixel portion can be formed in accordance with any ofthe above embodiments. The transistors can be easily formed as n-channeltransistors, and thus part of a driver circuit that can be formed usingan n-channel transistor can be formed over the same substrate as thetransistors in the pixel portion. By thus using the transistorsdescribed in the above embodiments for the pixel portion or the drivercircuit, a highly reliable display device can be provided.

FIG. 28A illustrates an example of a top view of an active matrixdisplay device. A pixel portion 701, a first scan line driver circuit702, a second scan line driver circuit 703, and a signal line drivercircuit 704 are formed over a substrate 700 of the display device. Inthe pixel portion 701, a plurality of signal lines extending from thesignal line driver circuit 704 are arranged and a plurality of scanlines extending from the first scan line driver circuit 702 and thesecond scan line driver circuit 703 are arranged. Note that pixels whichinclude display elements are provided in a matrix in the respectiveregions where the scan lines and the signal lines intersect with eachother. The substrate 700 of the display device is connected to a timingcontrol circuit (also referred to as a controller or a controller IC)through a connection portion such as a flexible printed circuit (FPC).

In FIG. 28A, the first scan line driver circuit 702, the second scanline driver circuit 703, and the signal line driver circuit 704 areformed over the substrate 700 where the pixel portion 701 is formed.Accordingly, the number of components which are provided outside, suchas a driver circuit, can be reduced, so that a reduction in cost can beachieved. Furthermore, if the driver circuit is provided outside thesubstrate 700, wirings would need to extend to increase the number ofwiring connections. When the driver circuit is provided over thesubstrate 700, the number of wiring connections can be reduced,resulting in improved reliability or yield.

<Liquid Crystal Display Device>

FIG. 28B illustrates an example of a circuit configuration of the pixel.Here, a pixel circuit that can be used in a pixel of a VA liquid crystaldisplay device is illustrated.

This pixel circuit can be applied to a structure in which one pixelincludes a plurality of pixel electrodes. The pixel electrodes areconnected to different transistors, and the transistors can be drivenwith different gate signals. Accordingly, signals applied to individualpixel electrodes in a multi-domain pixel can be controlledindependently.

A gate wiring 712 of a transistor 716 and a gate wiring 713 of atransistor 717 are separated so that different gate signals can besupplied thereto. In contrast, a source or drain electrode 714 thatfunctions as a data line is shared by the transistors 716 and 717. Thetransistor described in any of the above embodiments can be used asappropriate as the transistors 716 and 717. Thus, a highly reliableliquid crystal display device can be provided.

The shapes of a first pixel electrode electrically connected to thetransistor 716 and a second pixel electrode electrically connected tothe transistor 717 are described. The first pixel electrode and thesecond pixel electrode are separated. There is no particular limitationon the shapes of the first pixel electrode and the second pixelelectrode. For example, the first pixel electrode may have a V-likeshape.

A gate electrode of the transistor 716 is connected to the gate wiring712, and a gate electrode of the transistor 717 is connected to the gatewiring 713. When different gate signals are supplied to the gate wiring712 and the gate wiring 713, operation timings of the transistor 716 andthe transistor 717 can be varied. As a result, alignment of liquidcrystals can be controlled.

Further, a storage capacitor may be formed using a capacitor wiring 710,a gate insulating film functioning as a dielectric, and a capacitorelectrode electrically connected to the first pixel electrode or thesecond pixel electrode.

The multi-domain pixel includes a first liquid crystal element 718 and asecond liquid crystal element 719. The first liquid crystal element 718includes the first pixel electrode, a counter electrode, and a liquidcrystal layer therebetween. The second liquid crystal element 719includes the second pixel electrode, a counter electrode, and a liquidcrystal layer therebetween.

Note that a pixel circuit of the present invention is not limited tothat shown in FIG. 28B. For example, a switch, a resistor, a capacitor,a transistor, a sensor, a logic circuit, or the like may be added to thepixel illustrated in FIG. 28B.

<Organic EL Display Device>

FIG. 28C illustrates another example of a circuit configuration of thepixel. Here, a pixel structure of a display device including an organicEL element is shown.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodes andholes are injected from the other of the pair of electrodes, into alayer containing a light-emitting organic compound; thus, current flows.The electrons and holes are recombined, and thus, the light-emittingorganic compound is excited. The light-emitting organic compound returnsto a ground state from the excited state, thereby emitting light. Owingto such a mechanism, this light-emitting element is referred to as acurrent-excitation light-emitting element.

FIG. 28C illustrates an applicable example of a pixel circuit. Here, onepixel includes two n-channel transistors. Note that an oxidesemiconductor film of one embodiment of the present invention can beused for channel formation regions of the n-channel transistors.Further, digital time grayscale driving can be employed for the pixelcircuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving are described.

A pixel 720 includes a switching transistor 721, a driver transistor722, a light-emitting element 724, and a capacitor 723. A gate electrodeof the switching transistor 721 is connected to a scan line 726, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 721 is connected to a signal line 725, and a secondelectrode (the other of the source electrode and the drain electrode) ofthe switching transistor 721 is connected to a gate electrode of thedriver transistor 722. The gate electrode of the driver transistor 722is connected to a power supply line 727 through the capacitor 723, afirst electrode of the driver transistor 722 is connected to the powersupply line 727, and a second electrode of the driver transistor 722 isconnected to a first electrode (a pixel electrode) of the light-emittingelement 724. A second electrode of the light-emitting element 724corresponds to a common electrode 728. The common electrode 728 iselectrically connected to a common potential line formed over the samesubstrate.

As the switching transistor 721 and the driver transistor 722, thetransistor described in any of the above embodiments can be used asappropriate. In this manner, a highly reliable organic EL display devicecan be provided.

The potential of the second electrode (the common electrode 728) of thelight-emitting element 724 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 727, and can be, forexample, GND or 0 V. The high power supply potential and the low powersupply potential are set to be higher than or equal to the forwardthreshold voltage of the light-emitting element 724, and the differencebetween the potentials is applied to the light-emitting element 724,whereby current is supplied to the light-emitting element 724 so thatlight is emitted. The forward voltage of the light-emitting element 724refers to a voltage at which a desired luminance is obtained, andincludes at least a forward threshold voltage.

Note that the gate capacitance of the driver transistor 722 may be usedas a substitute for the capacitor 723, in which case the capacitor 723can be omitted. The gate capacitance of the driver transistor 722 may beformed between the channel formation region and the gate electrode.

Next, a signal input to the driver transistor 722 is described. In thecase of a voltage-input voltage driving method, a video signal forsurely turning on or off the driver transistor 722 is input to thedriver transistor 722. In order for the driver transistor 722 to operatein a linear region, a voltage higher than the voltage of the powersupply line 727 is applied to the gate electrode of the drivertransistor 722. Note that a voltage higher than or equal to the sum ofpower supply line voltage and the threshold voltage Vth of the drivertransistor 722 is applied to the signal line 725.

In the case of performing analog grayscale driving, a voltage higherthan or equal to the sum of the forward voltage of the light-emittingelement 724 and the threshold voltage Vth of the driver transistor 722is applied to the gate electrode of the driver transistor 722. A videosignal by which the driver transistor 722 is operated in a saturationregion is input, so that current is supplied to the light-emittingelement 724. In order for the driver transistor 722 to operate in asaturation region, the potential of the power supply line 727 is sethigher than the gate potential of the driver transistor 722. When ananalog video signal is used, it is possible to supply current to thelight-emitting element 724 in accordance with the video signal andperform analog grayscale driving.

Note that the configuration of the pixel circuit of the presentinvention is not limited to that shown in FIG. 28C. For example, aswitch, a resistor, a capacitor, a sensor, a transistor, a logiccircuit, or the like may be added to the pixel circuit illustrated inFIG. 28C.

In the case where the transistor shown in any of the above embodimentsis used for the circuits shown in FIGS. 28A to 28C, the source electrode(the first electrode) is electrically connected to the low potentialside and the drain electrode (the second electrode) is electricallyconnected to the high potential side. Furthermore, the potential of thefirst gate electrode may be controlled by a control circuit or the likeand the potential described above as an example, e.g., a potential lowerthan the potential applied to the source electrode, may be input to thesecond gate electrode through a wiring that is not illustrated.

For example, in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ a variety of modes or caninclude a variety of elements. A display element, a display device, alight-emitting element, or a light-emitting device includes, forexample, at least one of an electroluminescence (EL) element (e.g., anEL element including organic and inorganic materials, an organic ELelement, or an inorganic EL element), an LED (e.g., a white LED, a redLED, a green LED, or a blue LED), a transistor (a transistor that emitslight depending on current), an electron emitter, a liquid crystalelement, electronic ink, an electrophoretic element, a grating lightvalve (GLV), a plasma display panel (PDP), a display element using microelectro mechanical system (MEMS), a digital micromirror device (DMD), adigital micro shutter (DMS), MIRASOL (registered trademark), aninterferometric modulator display (IMOD) element, a MEMS shutter displayelement, an optical-interference-type MEMS display element, anelectrowetting element, a piezoelectric ceramic display, and a displayelement including a carbon nanotube. Other than the above, display mediawhose contrast, luminance, reflectivity, transmittance, or the like ischanged by electrical or magnetic effect may be included. Examples ofthe display device including an EL element include an EL display.Examples of the display device including an electron emitter include afield emission display (FED) and an SED-type flat panel display (SED:surface-conduction electron-emitter display). Examples of the displaydevice including a liquid crystal element include a liquid crystaldisplay (e.g., a transmissive liquid crystal display, a transflectiveliquid crystal display, a reflective liquid crystal display, adirect-view liquid crystal display, or a projection liquid crystaldisplay). Examples of the display device including electronic ink,Electronic Liquid Powder (registered trademark), or an electrophoreticelement include electronic paper. In the case of a transflective liquidcrystal display or a reflective liquid crystal display, some or all ofpixel electrodes function as reflective electrodes. For example, some orall of pixel electrodes are formed to contain aluminum, silver, or thelike. In such a case, a memory circuit such as an SRAM can be providedunder the reflective electrodes. This results in a further reduction inpower consumption.

At least part of this embodiment can be implemented in appropriatecombination with any of the other embodiments described in thisspecification.

Embodiment 8

In this embodiment, a display module including a semiconductor device ofone embodiment of the present invention will be described with referenceto FIG. 29.

In a display module 8000 illustrated in FIG. 29, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight unit 8007, a frame 8009, a printed board 8010, and a battery8011 are provided between an upper cover 8001 and a lower cover 8002.Note that the backlight unit 8007, the battery 8011, the touch panel8004, and the like are not provided in some cases.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and can be formed to overlap with the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 to form an optical touch panel. An electrode fora touch sensor may be provided in each pixel of the display panel 8006so that a capacitive touch panel is obtained.

The backlight unit 8007 includes a light source 8008. The light source8008 may be provided at an end portion of the backlight unit 8007 and alight diffusing plate may be used.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

The structure shown in this embodiment can be used in appropriatecombination with the structure shown in any of the other embodiments.

Embodiment 9

In this embodiment, examples of an electronic device that uses asemiconductor device of one embodiment of the present invention will bedescribed.

Specific examples of the electronic device that uses the semiconductordevice of one embodiment of the present invention are as follows:display devices of televisions, monitors, and the like, lightingdevices, desktop and laptop personal computers, word processors, imagereproduction devices which reproduce still images and moving imagesstored in recording media such as digital versatile discs (DVDs),portable CD players, radios, tape recorders, headphone stereos, stereos,table clocks, wall clocks, cordless phone handsets, transceivers, mobilephones, car phones, portable game machines, tablet terminals, large gamemachines such as pachinko machines, calculators, portable informationterminals, electronic notebooks, e-book readers, electronic translators,audio input devices, video cameras, digital still cameras, electricshavers, high-frequency heating appliances such as microwave ovens,electric rice cookers, electric washing machines, electric vacuumcleaners, water heaters, electric fans, hair dryers, air-conditioningsystems such as air conditioners, humidifiers, and dehumidifiers,dishwashers, dish dryers, clothes dryers, futon dryers, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, flashlights, electrical tools such as achain saw, smoke detectors, and medical equipment such as dialyzers.Other examples are as follows: industrial equipment such as guidelights, traffic lights, conveyor belts, elevators, escalators,industrial robots, power storage systems, and power storage devices forleveling the amount of power supply and smart grid. In addition, movingobjects and the like driven by fuel engines or electric motors usingpower from non-aqueous secondary batteries are also included in thecategory of electronic devices. Examples of the moving objects includeelectric vehicles (EV), hybrid electric vehicles (HEV) which includeboth an internal-combustion engine and a motor, plug-in hybrid electricvehicles (PHEV), tracked vehicles in which caterpillar tracks aresubstituted for wheels of these vehicles, motorized bicycles includingmotor-assisted bicycles, motorcycles, electric wheelchairs, golf carts,boats, ships, submarines, helicopters, aircrafts, rockets, artificialsatellites, space probes, planetary probes, and spacecrafts.

FIG. 30A illustrates an example of a portable game console including ahousing 901, a housing 902, a display portion 903, a display portion904, a microphone 905, a speaker 906, an operation key 907, a stylus908, and the like. Although the portable game console in FIG. 30A hasthe two display portions 903 and 904, the number of display portionsincluded in a portable game console is not limited to this. Any of thetransistors described in Embodiments 1 to 4 can be used as transistorsincluded in the display portions 903 and 904 and the like. Furthermore,any of the transistors described in Embodiments 1 to 4 can be used in aCPU, a memory device, or the like which is not illustrated in FIG. 30A.

FIG. 30B illustrates an example of a portable data terminal including afirst housing 911, a second housing 912, a first display portion 913, asecond display portion 914, a joint 915, an operation key 916, and thelike. The first display portion 913 is provided in the first housing911, and the second display portion 914 is provided in the secondhousing 912. The first housing 911 and the second housing 912 areconnected to each other with the joint 915, and the angle between thefirst housing 911 and the second housing 912 can be changed with thejoint 915. An image on the first display portion 913 may be switched inaccordance with the angle between the first housing 911 and the secondhousing 912 at the joint 915. A display device with a position inputfunction may be used as at least one of the first display portion 913and the second display portion 914. Note that the position inputfunction can be added by providing a touch panel in a display device.Alternatively, the position input function can be added by provision ofa photoelectric conversion element called a photosensor in a pixelportion of a display device. Any of the transistors described inEmbodiments 1 to 4 can be used as transistors included in the firstdisplay portion 913, the second display portion 914, and the like.Furthermore, any of the transistors described in Embodiments 1 to 4 canbe used in a CPU, a memory device, or the like which is not illustratedin FIG. 30B.

FIG. 30C illustrates an example of a laptop personal computer, whichincludes a housing 921, a display portion 922, a keyboard 923, apointing device 924, and the like. Any of the transistors described inEmbodiments 1 to 4 can be used as transistors included in the displayportion 922 and the like. Furthermore, any of the transistors describedin Embodiments 1 to 4 can be used in a CPU, a memory device, or the likewhich is not illustrated in FIG. 30C.

FIG. 30D illustrates an example of an electric refrigerator-freezerincluding a housing 931, a door for a refrigerator 932, a door for afreezer 933, and the like. Any of the transistors described inEmbodiments 1 to 4 can be used in a CPU, a memory device, or the likewhich is not illustrated in FIG. 30D.

FIG. 30E illustrates an example of a video camera, which includes afirst housing 941, a second housing 942, a display portion 943,operation keys 944, a lens 945, a joint 946, and the like. The operationkeys 944 and the lens 945 are provided for the first housing 941, andthe display portion 943 is provided for the second housing 942. Thefirst housing 941 and the second housing 942 are connected to each otherwith the joint 946, and the angle between the first housing 941 and thesecond housing 942 can be changed with the joint 946. Images displayedon the display portion 943 may be switched in accordance with the angleat the joint 946 between the first housing 941 and the second housing942. Any of the transistors described in Embodiments 1 to 4 can be usedas transistors included in the display portion 943 and the like.Furthermore, any of the transistors described in Embodiments 1 to 4 canbe used in a CPU, a memory device, or the like which is not illustratedin FIG. 30E.

FIG. 30F illustrates an example of an automobile including a car body951, wheels 952, a dashboard 953, lights 954, and the like. Any of thetransistors described in Embodiments 1 to 4 can be used in a CPU, amemory device, or the like which is not illustrated in FIG. 30F.

This embodiment can be combined with any of the other embodimentsdescribed in this specification as appropriate.

Example 1

This example shows the results of calculating the concentration ofinjected oxygen in the depth direction of an oxide semiconductor filminto which oxygen ions are injected. In this example, as the oxidesemiconductor film to which oxygen ions are injected, the oxidesemiconductor film 107 in contact with the gate insulating film 105illustrated in FIGS. 1A to 1C is assumed.

For the calculation, transport of ions in matter (TRIM) was used.

A sample used in this calculation has a structure in which a siliconoxide film and an oxide semiconductor film are sequentially stacked overa silicon wafer.

The silicon oxide film has an element atomic ratio of Si:O=1:2, athickness of 100 nm, and a density of 2.2 g/cm³. The oxide semiconductorfilm is an IGZO film with an element atomic ratio ofIn:Ga:Zn:O=1:3:4:10, a thickness of 20 nm, and a density of 5.91 g/cm³.As the ion species, oxygen atomic ions with molecular weight of 16 wereused. The dose was 1×10¹⁶ ions/cm².

FIG. 31 shows the calculation results under the conditions where theacceleration voltage in injecting the ion species was 2.5 kV, 5 kV, and7.5 kV. In FIG. 31, SiO₂ represents the silicon oxide film, andIGZO(134) represents the oxide semiconductor film.

In FIG. 31, the horizontal axis and the vertical axis indicate the depthdirection and the oxygen concentration, respectively. The solid linerepresents the calculation results at an acceleration voltage of 2.5 kV,the broken line represents the calculation results at an accelerationvoltage of 5 kV, and the dashed-dotted line represents the calculationresults at an acceleration voltage of 7.5 kV.

These results show that adjusting the acceleration voltage of the ionspecies and the thickness of the oxide semiconductor film allows thecontrol of the concentration of injected oxygen at the interface betweenthe silicon oxide film and the oxide semiconductor film. In addition,the amount of oxygen injected into the silicon oxide film can becontrolled.

Example 2

This example shows the results of calculating the concentration ofinjected oxygen in the depth direction of an oxide semiconductor filminto which oxygen ions are injected. In this example, as the oxidesemiconductor film to which oxygen ions are injected, the oxidesemiconductor film 115 in contact with the gate insulating film 117illustrated in FIGS. 1A to 1C is assumed.

For the calculation, TRIM was used.

A sample used in this calculation has a structure in which a siliconoxide film, a first oxide semiconductor film, a second oxidesemiconductor film, and a third oxide semiconductor film aresequentially stacked over a silicon wafer.

The silicon oxide film has an element atomic ratio of Si:O=1:2, athickness of 100 nm, and a density of 2.2 g/cm³. The first oxidesemiconductor film is an IGZO film with an element atomic ratio ofIn:Ga:Zn:O=1:3:4:10, a thickness of 20 nm, and a density of 5.91 g/cm³.The second oxide semiconductor film is an IGZO film with an elementatomic ratio of In:Ga:Zn:O=1:1:1:4, a thickness of 15 nm, and a densityof 6.24 g/cm³. The third oxide semiconductor film is an IGZO film withan element atomic ratio of In:Ga:Zn:O=1:3:2:8, a thickness of 5 nm, anda density of 5.71 g/cm³. As the ion species, oxygen atomic ions withmolecular weight of 16 were used. The dose was 1×10¹⁶ ions/cm².

FIG. 32 shows the calculation results under the conditions where theacceleration voltage in injecting the ion species was 2.5 kV, 5 kV, 7.5kV, 10 kV, and 15 kV. In FIG. 32, SiO₂ represents the silicon oxidefilm, IGZO(134) represents the first oxide semiconductor film, IGZO(111)represents the second oxide semiconductor film, and IGZO(132) representsthe third oxide semiconductor film.

In FIG. 32, the horizontal axis and the vertical axis indicate the depthdirection and the oxygen concentration, respectively. The thin solidline represents the calculation results at an acceleration voltage of2.5 kV, the thin broken line represents the calculation results at anacceleration voltage of 5 kV, and the thin dashed-dotted line representsthe calculation results at an acceleration voltage of 7.5 kV. The thicksolid line represents the calculation results at an acceleration voltageof 10 kV, and the thick broken line represents the calculation resultsat an acceleration voltage of 15 kV.

These results show that adjusting the acceleration voltage of the ionspecies and the thickness of the oxide semiconductor film allows thecontrol of the concentration of injected oxygen at the interface betweenthe silicon oxide film and the oxide semiconductor film. In addition,the amount of oxygen injected into the silicon oxide film can becontrolled.

Example 3

This example shows the results of measuring the number of defects in anoxide insulating film in contact with an oxide semiconductor film.

<Fabrication Method of Samples>

In this example, Sample A1 and Sample A2 were fabricated.

<Sample A1>

A fabrication method of Sample A1 is described with reference to FIGS.33A to 33D.

As illustrated in FIG. 33A, a 100-nm-thick silicon oxide film 303 wasformed over a quartz substrate 301, and a 40-nm-thick first IGZO film305 was formed over the silicon oxide film 303. Then, oxygen atomic ions306 were injected into the first IGZO film 305. As a result, a firstIGZO film 305 a to which the oxygen atomic ions had been added wasobtained as illustrated in FIG. 33B.

The silicon oxide film 303 was formed by a plasma CVD method under theconditions where 1 sccm of silane and 800 sccm of dinitrogen monoxidewere used as source gases; the pressure in a reaction chamber was 40 Pa;the substrate temperature was 500° C.; and a power of 150 W wassupplied.

The first IGZO film 305 was formed by a sputtering method under theconditions where an In—Ga—Zn oxide target with an atomic ratio ofIn:Ga:Zn=1:3:4 was used as a sputtering target; a gas containing 11% ofoxygen (diluted with argon) was used as a sputtering gas; the pressurein a reaction chamber was 0.7 Pa; the substrate temperature was 200° C.;and a DC power of 0.5 kW was supplied.

The oxygen atomic ions 306 were added to the first IGZO film 305 by anion implantation method at an acceleration voltage of 5 kV to a dose of1×10¹⁶ ions/cm².

Then, as illustrated in FIG. 33C, a 50-nm-thick second IGZO film 307 wasformed over the first IGZO film 305 a to which the oxygen atomic ionshad been added.

The second IGZO film 307 was formed by a sputtering method under theconditions where an In—Ga—Zn oxide target with an atomic ratio ofIn:Ga:Zn=1:1:1 was used as a sputtering target; a gas containing 11% ofoxygen (diluted with argon) was used as a sputtering gas; the pressurein a reaction chamber was 0.7 Pa; the substrate temperature was 300° C.;and a DC power of 0.5 kW was supplied.

Next, heat treatment was performed as illustrated in FIG. 33D. As aresult, part of oxygen contained in the first IGZO film 305 a wasdiffused into the second IGZO film 307. In FIG. 33D, the first IGZO filmand the second IGZO film after the heat treatment are denoted by a firstIGZO film 305 b and a second IGZO film 307 a, respectively.

Here, heat treatment was performed at 450° C. in a nitrogen atmospherefor 1 hour, and after that, another heat treatment was performed at 450°C. in an oxygen atmosphere for 1 hour.

Through the above steps, Sample A1 was fabricated.

<Sample A2>

A fabrication method of Sample A2 is described with reference to FIGS.33E to 33I. Note that Sample A2 is different from Sample A1 in thematerial for the substrate and the formation temperature of the siliconoxide film, and in that oxygen atomic ions are added to the siliconoxide film.

As illustrated in FIG. 33E, a 100-nm-thick silicon oxide film 312 wasformed on the surface of a silicon wafer 311 by heating the siliconwafer 311. Then, a 300-nm-thick silicon oxide film 313 was formed overthe silicon oxide film 312. After that, oxygen atomic ions 316 wereinjected into the silicon oxide film 313. As a result, a silicon oxidefilm 313 a to which the oxygen atomic ions had been added was obtainedas illustrated in FIG. 33F.

The silicon oxide film 312 was formed on the surface of the siliconwafer 311 by heating the silicon wafer 311 at 950° C. in a water vaporatmosphere.

The silicon oxide film 313 was formed by a plasma CVD method under theconditions where 2.3 sccm of silane and 800 sccm of dinitrogen monoxidewere used as source gases; the pressure in a reaction chamber was 40 Pa;the substrate temperature was 400° C.; and a power of 50 W was supplied.

The oxygen atomic ions 316 were added to the silicon oxide film 313 byan ion implantation method at an acceleration voltage of 60 kV to a doseof 2×10¹⁶ ions/cm².

Then, as illustrated in FIG. 33G, an 80-nm-thick first IGZO film 315 wasformed over the silicon oxide film 313 a. After that, a 15-nm-thicksecond IGZO film 317 was formed over the first IGZO film 315.

The first IGZO film 315 was formed by a sputtering method under theconditions where an In—Ga—Zn oxide target with an atomic ratio ofIn:Ga:Zn=1:3:2 was used as a sputtering target; a gas containing 11% ofoxygen (diluted with argon) was used as a sputtering gas; the pressurein a reaction chamber was 0.7 Pa; the substrate temperature was 200° C.;and a DC power of 0.5 kW was supplied.

The second IGZO film 317 was formed by a sputtering method under theconditions where an In—Ga—Zn oxide target with an atomic ratio ofIn:Ga:Zn=1:1:1 was used as a sputtering target; a gas containing 33% ofoxygen (diluted with argon) was used as a sputtering gas; the pressurein a reaction chamber was 0.7 Pa; the substrate temperature was 300° C.;and a DC power of 0.5 kW was supplied.

Next, heat treatment was performed. As a result, part of oxygencontained in the silicon oxide film 313 a was diffused into the firstIGZO film 315 and the second IGZO film 317. In FIG. 33H, the siliconoxide film, the first IGZO film, and the second IGZO film after the heattreatment are denoted by a silicon oxide film 313 b, a first IGZO film315 a, and a second IGZO film 317 a, respectively.

Here, heat treatment was performed at 450° C. in a nitrogen atmospherefor 1 hour, and after that, another heat treatment was performed at 450°C. in an oxygen atmosphere for 1 hour.

Then, as illustrated in FIG. 33I, the first IGZO film 315 a and thesecond IGZO film 317 a were removed.

Through the above steps, Sample A2 was fabricated.

<ESR Measurement>

Samples A1 and A2 were subjected to ESR measurement. Here, the ESRmeasurement was performed under the following conditions: for Sample A1,the measurement temperature was 10 K, the high-frequency power of 9.45GHz was 0.05 mW, and the direction of a magnetic field was parallel to asurface of the sample; for Sample A2, the measurement temperature was100 K, the high-frequency power of 9.15 GHz was 4 mW, and the directionof a magnetic field was parallel to a surface of the sample. The lowerdetection limit of the spin density of a signal attributed to NO is1.0×10¹⁷ spins/cm³ at a measurement temperature of 100 K. A smallernumber of spins means a smaller number of defects in the film.

FIGS. 34A and 34B show the measurement results of Sample A1 and SampleA2, respectively. The spin density of Sample A1 was lower than thedetection limit. In Sample A2, a first signal that appears at a g-factorof greater than or equal to 2.037 and less than or equal to 2.039, asecond signal that appears at a g-factor of greater than or equal to2.001 and less than or equal to 2.003, and a third signal that appearsat a g-factor of greater than or equal to 1.964 and less than or equalto 1.966 were observed. These three signals are due to NO and representsplits of a hyperfine structure arising from the nuclear spin of an Natom. The signals attributed to NO have anisotropic spin species andthus the waveform is asymmetrical. The spin density of Sample A2 was5.5×10¹⁷ spins/cm³.

FIGS. 34A and 34B indicate that the number of defects in the siliconoxide film serving as a base film of the oxide semiconductor filmincreases when oxygen atomic ions are added to the silicon oxide filmand the heat treatment is performed. In contrast, when oxygen atomicions are added to the oxide semiconductor film, the number of defects inthe silicon oxide film serving as a base film of the oxide semiconductorfilm decreases.

Described next is the relationship between the formation temperature ofthe silicon oxide film serving as a base film of the oxide semiconductorfilm and the number of defects in the silicon oxide film. Description isalso made on the number of defects in the silicon oxide film in relationto the addition of oxygen atomic ions to the oxide semiconductor film.

<Sample A3>

A fabrication method of Sample A3 is described with reference to FIGS.35A and 35B.

As illustrated in FIG. 35A, a 400-nm-thick silicon oxide film 322 wasformed on the surface of a silicon wafer 321 by heating the siliconwafer 321. Then, a 100-nm-thick silicon oxide film 323 was formed overthe silicon oxide film 322. After that, a 20-nm-thick first IGZO film325 was formed over the silicon oxide film 323. Then, a 20-nm-thicksecond IGZO film 327 was formed over the first IGZO film 325.

The silicon oxide film 322 was formed on the surface of the siliconwafer 321 by heating the silicon wafer 321 at 950° C. in a water vaporatmosphere.

The silicon oxide film 323 was formed by a plasma CVD method under theconditions where 1 sccm of silane and 800 sccm of dinitrogen monoxidewere used as source gases; the pressure in a reaction chamber was 40 Pa;the substrate temperature was 500° C.; and a power of 150 W wassupplied.

The first IGZO film 325 was formed by a sputtering method under theconditions where an In—Ga—Zn oxide target with an atomic ratio ofIn:Ga:Zn=1:3:4 was used as a sputtering target; a gas containing 11% ofoxygen (diluted with argon) was used as a sputtering gas; the pressurein a reaction chamber was 0.7 Pa; the substrate temperature was 200° C.;and a DC power of 0.5 kW was supplied.

The second IGZO film 327 was formed by a sputtering method under theconditions where an In—Ga—Zn oxide target with an atomic ratio ofIn:Ga:Zn=1:1:1 was used as a sputtering target; a gas containing 33% ofoxygen (diluted with argon) was used as a sputtering gas; the pressurein a reaction chamber was 0.7 Pa; the substrate temperature was 300° C.;and a DC power of 0.5 kW was supplied.

Next, heat treatment was performed. In FIG. 35B, the first IGZO film andthe second IGZO film after the heat treatment are denoted by a firstIGZO film 325 a and a second IGZO film 327 a, respectively.

Here, heat treatment was performed at 450° C. in a nitrogen atmospherefor 1 hour, and after that, another heat treatment was performed at 450°C. in an oxygen atmosphere for 1 hour.

Through the above steps, Sample A3 was fabricated.

<Sample A4>

The fabrication conditions of Sample A4 are different from those ofSample A3 in the formation temperature of the silicon oxide film 323.

The silicon oxide film 323 was formed by a plasma CVD method under theconditions where 1 sccm of silane and 800 sccm of dinitrogen monoxidewere used as source gases; the pressure in a reaction chamber was 40 Pa;the substrate temperature was 400° C.; and a power of 150 W wassupplied.

<Sample A5>

A fabrication method of Sample A5 is described with reference to FIGS.35C to 35F. Sample A5 is different from sample A3 in that oxygen atomicions are added to the first IGZO film.

As illustrated in FIG. 35C, the 400-nm-thick silicon oxide film 322 wasformed on the surface of the silicon wafer 321 by heating the siliconwafer 321. Then, the 100-nm-thick silicon oxide film 323 was formed overthe silicon oxide film 322. After that, the 20-nm-thick first IGZO film325 was formed over the silicon oxide film 323. Then, oxygen atomic ions326 were added to the first IGZO film 325. As a result, a first IGZOfilm 325 a to which the oxygen atomic ions had been added was obtainedas illustrated in FIG. 35D.

The silicon oxide film 322, the silicon oxide film 323, and the firstIGZO film 325 were formed under the same conditions as Sample A3. Thatis, the silicon oxide film 323 in Sample A5 was formed at a substratetemperature of 500° C.

The oxygen atomic ions 326 were added to the first IGZO film 325 by anion implantation method at an acceleration voltage of 5 kV to a dose of1×10¹⁶ ions/cm².

Then, as illustrated in FIG. 35E, the 20-nm-thick second IGZO film 327was formed over the first IGZO film 325 a.

The second IGZO film 327 was formed by a sputtering method under theconditions where an In—Ga—Zn oxide target with an atomic ratio ofIn:Ga:Zn=1:1:1 was used as a sputtering target; a gas containing 33% ofoxygen (diluted with argon) was used as a sputtering gas; the pressurein a reaction chamber was 0.7 Pa; the substrate temperature was 300° C.;and a DC power of 0.5 kW was supplied.

Next, heat treatment was performed. As a result, part of oxygencontained in the first IGZO film 325 a was diffused into the second IGZOfilm 327. In FIG. 35F, the first IGZO film and the second IGZO filmafter the heat treatment are denoted by a first IGZO film 325 b and asecond IGZO film 327 a, respectively.

Here, heat treatment was performed at 450° C. in a nitrogen atmospherefor 1 hour, and after that, another heat treatment was performed at 450°C. in an oxygen atmosphere for 1 hour.

Through the above steps, Sample A5 was fabricated.

<Sample A6>

The fabrication conditions of Sample A6 are different from those ofSample A5 in the formation temperature of the silicon oxide film 323.

The silicon oxide film 323 was formed by a plasma CVD method under theconditions where 1 sccm of silane and 800 sccm of dinitrogen monoxidewere used as source gases; the pressure in a reaction chamber was 40 Pa;the substrate temperature was 400° C.; and a power of 150 W wassupplied.

<ESR Measurement>

Next, Samples A3 to A6 were subjected to ESR measurement. Here, the ESRmeasurement was performed under the following conditions: themeasurement temperature was 10 K, the high-frequency power of 9.55 GHzwas 0.1 mW, and the direction of a magnetic field was parallel to asurface of the sample. The lower detection limit of the spin density ofa signal attributed to NO is 8.7×10¹⁵ spins/cm³ at a measurementtemperature of 10 K.

FIG. 36A shows the measurement results of Samples A3 and A4, and FIG.36B shows the measurement results of Samples A5 and A6. FIGS. 36A and36B also show the spin densities of the three signals attributed to NOin Samples A3 to A6. The spin density in Sample A3 was lower than thedetection limit as shown in FIG. 36A. The spin density in Sample A4 was8.7×10¹⁵ spins/cm³. The spin density in Sample A5 was 7.2×10¹⁶spins/cm³. The spin density in Sample A6 was 1.6×10¹⁷ spins/cm³. Asshown in FIG. 36B, the three signals attributed to NO were observed ineach of Samples A5 and A6.

FIG. 37 shows the spin densities of the three signals attributed to NOin Samples A3 to A6.

FIGS. 36A and 36B and FIG. 37 indicate that when the silicon oxide filmserving as a base film of the first IGZO film is formed at a highertemperature, the number of defects in the silicon oxide film can bereduced.

FIGS. 36A and 36B and FIG. 37 also indicate that defects are generatedin the silicon oxide film serving as a base film when oxygen atomic ionsare added to the oxide semiconductor film.

Example 4

Described in this example are the relationship between the formationtemperature of an oxide insulating film and the amount of water releasedfrom the film by heat treatment, and the relationship between theformation temperature of the oxide insulating film and the nitrogenconcentration of the film.

<Fabrication Method of Samples>

In this example, Sample B1 to Sample B5 were fabricated.

<Sample B1>

A 100-nm-thick silicon oxide film was formed on the surface of a siliconwafer by heating the silicon wafer. Then, a 100-nm-thick silicon oxidefilm was formed over the silicon oxide film.

The silicon oxide film was formed on the surface of the silicon wafer byheating the silicon wafer at 950° C. in an oxygen atmosphere to whichHCl had been added.

The silicon oxide film was formed by a plasma CVD method under theconditions where 1 sccm of silane and 800 sccm of dinitrogen monoxidewere used as source gases; the pressure in a reaction chamber was 40 Pa;the substrate temperature was 350° C.; and a power of 150 W wassupplied.

Through the above steps, Sample B1 was fabricated.

<Sample B2>

Sample B2 was fabricated in a manner similar to that of Sample B1 exceptthat the silicon oxide film was formed at 400° C.

<Sample B3>

Sample B3 was fabricated in a manner similar to that of Sample B1 exceptthat the silicon oxide film was formed at 450° C.

<Sample B4>

Sample B4 was fabricated in a manner similar to that of Sample B1 exceptthat the silicon oxide film was formed at 500° C.

<Sample B5>

Sample B5 was fabricated in a manner similar to that of Sample B1 exceptthat the silicon oxide film was formed at 550° C.

<TDS Measurement>

Next, Samples B1 to B5 were subjected to TDS analysis.

The peaks of the curves shown in the results obtained from TDS analysisappear due to release of atoms or molecules contained in the analyzedsamples (in this example, Samples B1 to B5) to the outside. The totalamount of the atoms or molecules released to the outside corresponds tothe integral value of the peak. Thus, with the degree of the peakintensity, the total amount of the atoms or molecules contained in thesilicon oxynitride film can be measured.

FIG. 38 shows the results of the TDS analysis on Samples B1 to B5. FIG.38 is a graph showing the amount of released water with respect to thesubstrate temperature.

FIG. 38 shows that the amount of released water increases as the heattreatment temperature rises even when the silicon oxide film is formedat a high temperature. In particular, the amount of released water islikely to increase when the heat treatment is performed at a temperaturehigher than the formation temperature of the silicon oxide film.

<SIMS Analysis>

Next, the silicon oxide films included in Samples B1 to B5 weresubjected to SIMS analysis. Note that here, the silicon oxide film wasnot formed by heating the silicon wafer, but a 100-nm-thick siliconoxide film was formed on a silicon wafer under the conditions of SampleB1 to B5. Samples including the silicon oxide films formed under therespective conditions of Samples B1 to B5 are referred to as Samples B1ato B5a. In each of the samples, the concentration of nitrogen wasmeasured from the silicon wafer (Si) to the silicon oxide film (SiON).FIG. 39 shows the measurement results of Samples B1a to B5a.

In FIG. 39, the horizontal axis represents the distance in the depthdirection and the vertical axis represents the nitrogen concentration.

The nitrogen concentration of Sample B1a was 6×10²⁰ atoms/cm³; SampleB2a, 4×10²⁰ atoms/cm³; Sample B3a, 2×10²⁰ atoms/cm³; Sample B4a, 7×10¹⁹atoms/cm³; and Sample B5a, 2×10¹⁹ atoms/cm³.

The hydrogen concentrations of Samples B1a to B5a were also measured. Ineach of the samples, the concentration of hydrogen was measured from thesilicon wafer (Si) to the silicon oxide film (SiON). FIG. 46 shows themeasurement results of Samples B1a to B5a.

In FIG. 46, the horizontal axis represents the distance in the depthdirection and the vertical axis represents the hydrogen concentration.

The hydrogen concentration of Sample B1a was 7×10²⁰ atoms/cm³, thehydrogen concentration of Sample B2a was 6×10²⁰ atoms/cm³, the hydrogenconcentration of Sample B3a was 5×10²⁰ atoms/cm³, the hydrogenconcentration of Sample B4a was 4×10¹⁹ atoms/cm³, and the hydrogenconcentration of Sample B5a was 3×10¹⁹ atoms/cm³.

It is known that it is difficult to obtain accurate data in theproximity of a surface of a sample or in the proximity of an interfacebetween stacked films formed using different materials by the SIMSanalysis in measurement principle. Thus, in the case where distributionsof the concentrations of nitrogen in the film in the thickness directionare analyzed by SIMS, an average value in a region where the film isprovided, the value is not greatly changed, and an almost constant levelof strength can be obtained is employed as the concentrations ofnitrogen.

FIG. 39 shows that the nitrogen concentration of the silicon oxide filmdecreases as the formation temperature thereof rises. FIG. 46 shows thatthe hydrogen concentration of the silicon oxide film decreases as theformation temperature thereof rises. The comparison between FIG. 39 andFIG. 46 suggests that as the formation temperature rises, the nitrogenconcentration decreases more significantly than the hydrogenconcentration.

Example 5

This example shows the fabrication of transistors and the measurementresults of the electrical characteristics of the transistors.

<Fabrication Method of Sample C1>

First, a fabrication process of Sample C1 including a transistor will bedescribed. In this example, a method for fabricating the transistor isdescribed with reference to FIGS. 3A to 3D, FIGS. 6A to 6C, and FIGS. 7Ato 7D.

As illustrated in FIG. 3A, an insulating film (not illustrated) wasformed over the substrate 101, and the gate electrode 103 was formedover the insulating film. Next, the insulating film 104 was formed overthe insulating film and the gate electrode 103, and the oxidesemiconductor film 106 was formed over the insulating film 104. Next,the oxygen 108 was added to the oxide semiconductor film 106, so thatthe oxide semiconductor film 106 a to which oxygen was added was formedas illustrated in FIG. 3B.

A silicon wafer was used as the substrate 101.

The substrate 101 was heated at 950° C. in a water vapor atmosphere, sothat a 400-nm-thick silicon oxide film was formed as the insulating filmon the surface of the substrate 101.

Furthermore, a 50-nm-thick tungsten film was formed by a sputteringmethod over the insulating film, a mask was formed over the tungstenfilm by a photolithography process, and the tungsten film wasselectively etched, so that the gate electrode 103 was formed. Then, themask was removed.

As the insulating film 104, a 100-nm-thick silicon oxide film was formedby a plasma CVD method.

The silicon oxide film was formed under the conditions where 1 sccm ofsilane and 800 sccm of dinitrogen monoxide were used as source gases;the pressure in a reaction chamber was 40 Pa; the substrate temperaturewas 500° C.; and a power of 150 W was supplied.

As the oxide semiconductor film 106, a 40-nm-thick In—Ga—Zn oxide filmwas formed by a sputtering method. The sputtering conditions at thistime were as follows: a target with a ratio of In:Ga:Zn=1:3:4 was used,11% of oxygen was introduced as a sputtering gas into a chamber having apressure of 0.7 Pa, the substrate temperature was 200° C., and a powerof 0.5 kW was supplied.

Oxygen molecular ions were added as the oxygen 108 by an ionimplantation method at an acceleration voltage of 5 kV to a dose of1×10¹⁶ ions/cm².

Next, as illustrated in FIG. 3B, the oxide semiconductor film 109 wasformed over the oxide semiconductor film 106 a to which oxygen wasadded.

As the oxide semiconductor film 109, a 20-nm-thick In—Ga—Zn oxide filmwas formed by a sputtering method. The sputtering conditions at thistime were as follows: a target with a ratio of In:Ga:Zn=1:1:1 was used,33% of oxygen was introduced as a sputtering gas into a chamber having apressure of 0.7 Pa, the substrate temperature was 300° C., and a powerof 0.5 kW was supplied.

Then, heat treatment was performed so that part of oxygen contained inthe oxide semiconductor film 106 a was transferred to the oxidesemiconductor film 109, so that the oxide semiconductor film 106 b andthe oxide semiconductor film 109 a in which oxygen vacancies werereduced were formed as illustrated in FIG. 3C. For Sample C1, the heattreatment was performed at a temperature lower than the formationtemperature of the insulating film 104.

Here, heat treatment was performed at 450° C. in a nitrogen atmospherefor 1 hour, and after that, another heat treatment was performed at 450°C. in an oxygen atmosphere for 1 hour.

Next, as illustrated in FIG. 8A, the conductive film 112 was formed overthe oxide semiconductor film 109 a.

Here, as the conductive film 112, a 100-nm-thick tungsten film wasformed by a sputtering method.

Next, a mask was formed over the conductive film 112 by aphotolithography process, and then the insulating film 104, the oxidesemiconductor film 106 a, the oxide semiconductor film 109 a, and theconductive film 112 were etched; thus, the gate insulating film 105, theoxide semiconductor film 107, the oxide semiconductor film 110, and theconductive film 113 were formed as illustrated in FIG. 8B.

Next, an oxide semiconductor film, an insulating film, and a conductivefilm were stacked over the pair of electrodes 113 a and 113 b and theoxide semiconductor film 111. Then, a mask was formed over theconductive film by a photolithography process, and the oxidesemiconductor film, the insulating film, and the conductive film wereetched to form the oxide semiconductor film 115, the gate insulatingfilm 117, and the gate electrode 119 as illustrated in FIGS. 7A to 7D.

As the oxide semiconductor film to be the oxide semiconductor film 115,a 5-nm-thick In—Ga—Zn oxide film was formed by a sputtering method. Thesputtering conditions at this time were as follows: a target with aratio of In:Ga:Zn=1:3:2 was used, 33% of oxygen was introduced as asputtering gas into a chamber having a pressure of 0.4 Pa, the substratetemperature was 200° C., and a power of 0.5 kW was supplied.

As the insulating film to be the gate insulating film 117, a 20-nm-thicksilicon oxynitride film was formed by a plasma CVD method.

As the conductive film to be the gate electrode 119, a 30-nm-thicktitanium nitride film and a 135-nm-thick tungsten film were stacked by asputtering method.

Next, the insulating film 121 was formed, and heat treatment wasperformed to form the insulating film 123.

As the insulating film 121, a 140-nm-thick aluminum oxide film wasformed by a sputtering method.

The heat treatment was performed at 400° C. in an oxygen atmosphere for1 hour.

As the insulating film 123, a 300-nm-thick silicon oxide film was formedby a plasma CVD method.

Next, a mask was formed over the insulating film 121 and the insulatingfilm 123 by a photolithography process, part of the insulating film 121and part of the insulating film 123 were etched to expose part of thepair of electrodes 113 a and 113 b, and the plugs 127 a and 127 billustrated in FIGS. 7A to 7D were formed.

Here, as the plugs 127 a and 127 b, a 50-nm-thick titanium film, a300-nm-thick aluminum film, and a 5-nm-thick titanium film were stackedby a sputtering method.

Through the above steps, the transistor was fabricated. Note that thetransistor was designed to have a channel length of 0.8 μm and a channelwidth of 0.8 μm.

<Fabrication Method of Sample C2>

A transistor in Sample C2 was fabricated in a manner similar to that ofthe transistor in Sample C1 except that heat treatment after theformation of the oxide semiconductor film 109 was performed at 550° C.In other words, for Sample C2, the heat treatment was performed at atemperature higher than the formation temperature of the insulating film104.

<Measurement Results of Electrical Characteristics>

Next, the electrical characteristics of the transistors were measured.First, the electrical characteristics before a stress test (hereinafterreferred to as initial characteristics) were measured. Thecharacteristics measured here were the V_(g)−Id characteristics, namely,a source-drain current (hereinafter referred to as a drain current) thatchanged when a source-gate voltage (hereinafter referred to as a gatevoltage) changed from −3 V to +3 V at a source-drain voltage(hereinafter referred to as a drain voltage) of 0.1 V or 1.8 V.

Next, a stress test, a BT stress test here, was performed on thetransistor.

The +GBT stress test was carried out under the conditions where thevoltage (V_(g)) of the gate electrode 119 was +3.3 V; the voltage(V_(d)) of the electrode 113 c was 0 V; the voltage (V_(s)) of theelectrode 113 d was 0 V; the voltage (V_(bg)) of the gate electrode 103was 0 V; the substrate temperature was 150° C.; and the stress time was1 hour. Then, the V_(g)−Id characteristics of the transistors weremeasured.

The −GBT stress test was carried out under the conditions where thevoltage (V_(g)) of the gate electrode 119 was −1.8 V; the voltage(V_(d)) of the electrode 113 c was 0 V; the voltage (V_(s)) of theelectrode 113 d was 0 V; the voltage (V_(bg)) of the gate electrode 103was 0 V; the substrate temperature was 85° C.; and the stress time was 1hour. Then, the V_(g)−Id characteristics of the transistors weremeasured.

The threshold voltage and the shift value in this specification will bedescribed. The threshold voltage (V_(th)) is defined as, in the V_(g)−Idcurve where the horizontal axis represents gate voltage (V_(g) [V]) andthe vertical axis represents the square root of drain current (Id^(1/2)[A^(1/2)]), a gate voltage at the intersection point of the line ofId^(1/2)=0 (V_(g) axis) and the tangent to the curve at a point wherethe slope of the curve is the steepest. Note that here, the thresholdvoltage is calculated with a drain voltage V_(d) of 1.8 V.

Furthermore, the shift value (Shift) in this specification is definedas, in the V_(g)−Id curve where the horizontal axis represents the gatevoltage (V_(g) [V]) and the vertical axis represents the logarithm ofthe drain current (I_(d) [A]), a gate voltage at the intersection pointof the line of I_(d)=1.0×10⁻¹² [A] and the tangent to the curve at apoint where the slope of the curve is the steepest. Note that here, theshift value is calculated with a drain voltage V_(d) of 1.8 V.

FIG. 40 shows the measurement results of the V_(g)−Id characteristics ofthe transistor in Sample C1 between before and after the +GBT stresstest and between before and after the −GBT stress test. FIG. 41 showsthe results of the +GBT stress test and the −GBT stress test on SampleC2.

In FIG. 40 and FIG. 41, the horizontal axis represents the voltage ofthe gate electrode 119 and the vertical axis represents the draincurrent. The initial characteristics are denoted by the dashed lines andthe V_(g)−Id characteristics after the stress tests are denoted by thesolid lines.

FIG. 40 indicates that the transistor in Sample C1 has normally-offcharacteristics. FIG. 40 also shows a small amount of change in thethreshold voltage and the shift value between before and after the +GBTstress test and the −GBT stress test.

In contrast, FIG. 41 shows that the transistor in Sample C2 has a largeamount of change in the threshold voltage between before and after the+GBT stress test, though it has normally-off characteristics.

Next, the time stress was applied to Sample C1 was extended to 60 hours.FIGS. 42A and 42B show the amount of change in the threshold voltage(ΔVth) and the shift value (ΔShift) between before and after the +GBTstress test.

The transistor included in Sample C1 has a small change in the thresholdvoltage and the shift value even after the long hours of +GBT stresstest; typically, greater than or equal to −0.1 V and less than or equalto 0.1 V. This suggests that high-temperature deposition of the siliconoxide film, which serves as a base film of the oxide semiconductor film,and addition of oxygen to the oxide semiconductor film lead to a smallvariation in the electrical characteristics of the transistor.

The variation in the electrical characteristics of the transistor canalso be reduced when the silicon oxide film serving as a base film ofthe oxide semiconductor film is formed at a temperature higher than thatof heat treatment performed on the oxide semiconductor film and oxygenis added to the oxide semiconductor film.

Example 3 shows that high-temperature formation of the silicon oxidefilm, which serves as a base film of the oxide semiconductor film,allows reducing the nitrogen concentration of the silicon oxide film.Furthermore, even when heat treatment is performed after oxygen is addedto the oxide semiconductor film over the silicon oxide film serving as abase film, generation of NO_(x) and an increase in defects in thesilicon oxide film can be suppressed.

The silicon oxide film formed at a high temperature has a decreasednitrogen concentration. Hence, even when oxygen is added to the siliconoxide film serving as a base film of the oxide semiconductor film byaddition of oxygen to the oxide semiconductor film, a nitrogen-oxygenbond is unlikely to occur in the silicon oxide film. Accordingly,electron traps due to NO are not easily generated in a back channel ofthe transistor, reducing the amount of change in the threshold voltageof the transistor due to the GBT stress test.

Example 4 shows that a large amount of water is released from thesilicon oxide film serving as a base film of the oxide semiconductorfilm when heat treatment is performed at 550° C. as compared with thecase where heat treatment is performed at 450° C. The amount of releasedwater also tends to increase when heat treatment is performed at atemperature higher than the formation temperature of the silicon oxidefilm. These results suggest that when the oxide semiconductor film isheated at 550° C., water might be diffused from the silicon oxide filminto the oxide semiconductor film or might be generated in the siliconoxide film. It is also suggested that when the oxide semiconductor filmis heated at a temperature higher than the formation temperature of thesilicon oxide film, water might be diffused from the silicon oxide filminto the oxide semiconductor film or might be generated in the siliconoxide film.

In Sample C1 of this example, heat treatment is performed at atemperature lower than the formation temperature of the silicon oxidefilm serving as a base film of the oxide semiconductor film.Furthermore, the transistor in Sample C1 has a small amount of change inthe threshold voltage due to the GBT stress test. These results indicatethat when the oxide semiconductor film is heated at a temperature lowerthan the formation temperature of the silicon oxide film serving as abase film of the oxide semiconductor film, electron traps caused bywater are not generated easily and the amount of change in the thresholdvoltage of the transistor due to the GBT stress test can be reduced.

EXPLANATION OF REFERENCE

11: oxide semiconductor film, 17: oxide semiconductor film, 100:transistor, 100 a: transistor, 100 b: transistor, 100 c: transistor, 100d: transistor, 100 e: transistor, 100 f: transistor, 100 g: transistor,100 h: transistor, 100 j: transistor, 101: substrate, 103: gateelectrode, 104: insulating film, 105: gate insulating film, 106: oxidesemiconductor film, 106 a: oxide semiconductor film, 106 b: oxidesemiconductor film, 107: oxide semiconductor film, 108: oxygen, 109:oxide semiconductor film, 109 a: oxide semiconductor film, 110: oxidesemiconductor film, 110 a: oxide semiconductor film, 111: oxidesemiconductor film, 111 a: oxide semiconductor film, 111 b: oxidesemiconductor film, 111 c: oxide semiconductor film, 111 e: impurityregion, 111 f: impurity region, 112: conductive film, 113: conductivefilm, 113 a: electrode, 113 b: electrode, 113 c: electrode, 113 d:electrode, 113 g: electrode, 113 h: electrode, 114: oxide semiconductorfilm, 114 a: oxide semiconductor film, 114 b: oxide semiconductor film,115: oxide semiconductor film, 115 a: oxide semiconductor film, 115 b:oxide semiconductor film, 115 c: oxide semiconductor film, 115 d: oxidesemiconductor film, 115 e: oxide semiconductor film, 116: insulatingfilm, 116 a: insulating film, 116 b: insulating film, 117: gateinsulating film, 117 a: gate insulating film, 117 b: gate insulatingfilm, 119: gate electrode, 119 a: gate electrode, 121: insulating film,123: insulating film, 125 a: opening, 125 b: opening, 127 a: plug, 127b: plug, 133 a: low-resistance region, 133 b: low-resistance region,134: Cu—X alloy film, 135: covering film, 136: conductive film, 137:Cu—X alloy film, 138: covering film, 200: transistor, 203: conductivefilm, 204: conductive film, 205: insulating film, 206: insulating film,215: insulating film, 216: plug, 230: capacitor, 301: quartz substrate,303: silicon oxide film, 305: IGZO film, 305 a: IGZO film, 305 b: IGZOfilm, 306: oxygen atomic ions, 307: IGZO film, 307 a: IGZO film, 311:silicon wafer, 312: silicon oxide film, 313: silicon oxide film, 313 a:silicon oxide film, 313 b: silicon oxide film, 315: IGZO film, 315 a:IGZO film, 316: oxygen atomic ions, 317: IGZO film, 317 a: IGZO film,321: silicon wafer, 322: silicon oxide film, 323: silicon oxide film,324: IGZO film, 325: IGZO film, 325 a: IGZO film, 325 b: IGZO film, 326:oxygen atomic ions, 327: IGZO film, 327 a: IGZO film, 700: substrate,701: pixel portion, 702: scan line driver circuit, 703: scan line drivercircuit, 704: signal line driver circuit, 710: capacitor wiring, 712:gate wiring, 713: gate wiring, 714: drain electrode, 716: transistor,717: transistor, 718: liquid crystal element, 719: liquid crystalelement, 720: pixel, 721: switching transistor, 722: driver transistor,723: capacitor, 724: light-emitting element, 725: signal line, 726: scanline, 727: power supply line, 728: common electrode, 750: transistor,751: gate electrode, 752: gate insulating film, 753: channel formationregion, 754: p-type impurity region, 755: p-type impurity region, 756:sidewall insulating film, 760: memory cell, 770: transistor, 789:element separation region, 790: insulating film, 791: insulating film,901: housing, 902: housing, 903: display portion, 904: display portion,905: microphone, 906: speaker, 907: operation key, 908: stylus, 911:housing, 912: housing, 913: display portion, 914: display portion, 915:joint, 916: operation key, 921: housing, 922: display portion, 923:keyboard, 924: pointing device, 931: housing, 932: door for arefrigerator, 933: door for a freezer, 941: housing, 942: housing, 943:display portion, 944: operation key, 945: lens, 946: joint, 951: carbody, 952: wheels, 953: dashboard, 954: light, 5100: pellet, 5120:substrate, 5161: region, 8000: display module, 8001: upper cover, 8002:lower cover, 8003: FPC, 8004: touch panel, 8005: FPC, 8006: displaypanel, 8007: backlight unit, 8008: light source, 8009: frame, 8010:printed board, 8011: battery.

This application is based on Japanese Patent Application serial No.2014-107570 filed with Japan Patent Office on May 23, 2014, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A method for manufacturing a semiconductordevice comprising the steps of: forming a first gate electrode over asubstrate; forming a first insulating film in contact with the firstgate electrode while heating the substrate at a temperature higher thanor equal to 450° C. and lower than a strain point of the substrate;forming a first oxide semiconductor film in contact with the firstinsulating film; adding oxygen to the first oxide semiconductor film;forming a second oxide semiconductor film in contact with the firstoxide semiconductor film after adding the oxygen to the first oxidesemiconductor film; performing a first heat treatment after forming thesecond oxide semiconductor film; etching part of the first insulatingfilm, part of the first oxide semiconductor film, and part of the secondoxide semiconductor film, whereby forming a first gate insulating filmhaving a projection, an etched first oxide semiconductor film, and anetched second oxide semiconductor film; forming a pair of electrodes incontact with the etched second oxide semiconductor film; forming a thirdoxide semiconductor film in contact with the etched second oxidesemiconductor film and the pair of electrodes; forming a second gateinsulating film in contact with the third oxide semiconductor film; andforming a second gate electrode in contact with the second gateinsulating film.
 2. The method for manufacturing the semiconductordevice according to claim 1, wherein the oxygen is added to the firstoxide semiconductor film by an ion implantation method, an ion dopingmethod, or a plasma treatment.
 3. The method for manufacturing thesemiconductor device according to claim 1, wherein after the formationof the third oxide semiconductor film, oxygen is added to the thirdoxide semiconductor film, and then a second heat treatment is performed,and wherein the second gate insulating film is formed in contact withthe heated third oxide semiconductor film.
 4. The method formanufacturing the semiconductor device according to claim 3, wherein atemperature of the second heat treatment is lower than a temperature atwhich the first insulating film is formed.
 5. The method formanufacturing the semiconductor device according to claim 1, wherein atemperature of the first heat treatment is lower than a temperature atwhich the first insulating film is formed.
 6. The method formanufacturing the semiconductor device according to claim 1, whereineach of the first oxide semiconductor film, the second oxidesemiconductor film, and the third oxide semiconductor film contains Inor Ga.
 7. The method for manufacturing the semiconductor deviceaccording to claim 1, wherein an energy level at a conduction bandminimum of each of the first and third oxide semiconductor films iscloser to a vacuum level than that of the second oxide semiconductorfilm is.
 8. The method for manufacturing the semiconductor deviceaccording to claim 1, wherein a difference between an energy level at aconduction band minimum of the second oxide semiconductor film and anenergy level at a conduction band minimum of each of the first and thirdoxide semiconductor films is greater than or equal to 0.05 eV and lessthan or equal to 2 eV.
 9. A method for manufacturing a semiconductordevice comprising the steps of: forming a first gate electrode over asubstrate; forming a first insulating film in contact with the firstgate electrode while heating the substrate at a temperature higher thanor equal to 450° C. and lower than a strain point of the substrate;forming a first oxide semiconductor film in contact with the firstinsulating film; adding oxygen to the first oxide semiconductor film;forming a second oxide semiconductor film in contact with the firstoxide semiconductor film after adding the oxygen to the first oxidesemiconductor film; performing a first heat treatment after forming thesecond oxide semiconductor film; forming a conductive film over thesecond oxide semiconductor film after performing the first heattreatment; etching part of the first insulating film, part of the firstoxide semiconductor film, part of the second oxide semiconductor film,and part of the conductive film, whereby forming a first gate insulatingfilm having a projection, an etched first oxide semiconductor film, anetched second oxide semiconductor film, and an etched conductive film;forming a pair of electrodes by etching the etched conductive film;forming a third oxide semiconductor film in contact with the etchedsecond oxide semiconductor film and the pair of electrodes; forming asecond gate insulating film in contact with the third oxidesemiconductor film; and forming a second gate electrode in contact withthe second gate insulating film.
 10. The method for manufacturing thesemiconductor device according to claim 9, wherein the oxygen is addedto the first oxide semiconductor film by an ion implantation method, anion doping method, or a plasma treatment.
 11. The method formanufacturing the semiconductor device according to claim 9, whereinafter the formation of the third oxide semiconductor film, oxygen isadded to the third oxide semiconductor film, and then a second heattreatment is performed, and wherein the second gate insulating film isformed in contact with the heated third oxide semiconductor film. 12.The method for manufacturing the semiconductor device according to claim11, wherein a temperature of the second heat treatment is lower than atemperature at which the first insulating film is formed.
 13. The methodfor manufacturing the semiconductor device according to claim 9, whereina temperature of the first heat treatment is lower than a temperature atwhich the first insulating film is formed.
 14. The method formanufacturing the semiconductor device according to claim 9, whereineach of the first oxide semiconductor film, the second oxidesemiconductor film, and the third oxide semiconductor film contains Inor Ga.
 15. The method for manufacturing the semiconductor deviceaccording to claim 9, wherein an energy level at a conduction bandminimum of each of the first and third oxide semiconductor films iscloser to a vacuum level than that of the second oxide semiconductorfilm is.
 16. The method for manufacturing the semiconductor deviceaccording to claim 9, wherein a difference between an energy level at aconduction band minimum of the second oxide semiconductor film and anenergy level at a conduction band minimum of each of the first and thirdoxide semiconductor films is greater than or equal to 0.05 eV and lessthan or equal to 2 eV.